diff --git a/CHANGELOG.md b/CHANGELOG.md
index e1e75dbe..153d5165 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Make custom directives definable via `customDirectives` property ([#124](https://github.com/marp-team/marpit/issues/124), [#125](https://github.com/marp-team/marpit/pull/125))
+
 ## v0.6.1 - 2019-01-25
 
 ### Fixed
diff --git a/src/markdown/directives/apply.js b/src/markdown/directives/apply.js
index 85c0bff2..524c335b 100644
--- a/src/markdown/directives/apply.js
+++ b/src/markdown/directives/apply.js
@@ -1,30 +1,30 @@
 /** @module */
 import kebabCase from 'lodash.kebabcase'
-import { globals, locals } from './directives'
+import builtInDirectives from './directives'
 import InlineStyle from '../../helpers/inline_style'
 
-const publicDirectives = [...Object.keys(globals), ...Object.keys(locals)]
-
 /**
  * Apply parsed Marpit directives to markdown-it tokens.
  *
  * @alias module:markdown/directives/apply
  * @param {MarkdownIt} md markdown-it instance.
+ * @param {Marpit} marpit Marpit instance.
  * @param {Object} [opts]
  * @param {boolean} [opts.dataset=true] Assigns directives as HTML data
  *     attributes of each section tag.
  * @param {boolean} [opts.css=true] Assigns directives as CSS Custom Properties
  *     of each section tag.
- * @param {boolean} [opts.includeInternal=false] Whether include internal
- *     directives (Undefined in {@link module:markdown/directives/directives}.)
- *     In default, internal directives are not applied to HTML/CSS.
  */
-function apply(md, opts = {}) {
+function apply(md, marpit, opts = {}) {
   const dataset = opts.dataset === undefined ? true : !!opts.dataset
   const css = opts.css === undefined ? true : !!opts.css
 
-  const filterFunc = key =>
-    !!opts.includeInternal || publicDirectives.includes(key)
+  const { global, local } = marpit.customDirectives
+  const directives = [
+    ...Object.keys(global),
+    ...Object.keys(local),
+    ...builtInDirectives,
+  ]
 
   md.core.ruler.after(
     'marpit_directives_parse',
@@ -39,7 +39,7 @@ function apply(md, opts = {}) {
           const style = new InlineStyle(token.attrGet('style'))
 
           for (const dir of Object.keys(marpitDirectives)) {
-            if (filterFunc(dir)) {
+            if (directives.includes(dir)) {
               const value = marpitDirectives[dir]
 
               if (value) {
diff --git a/src/markdown/directives/directives.js b/src/markdown/directives/directives.js
index c86e0760..93523e89 100644
--- a/src/markdown/directives/directives.js
+++ b/src/markdown/directives/directives.js
@@ -24,7 +24,7 @@
  * @prop {Directive} theme Specify theme of the slide deck.
  */
 export const globals = {
-  headingDivider(value) {
+  headingDivider: value => {
     const headings = [1, 2, 3, 4, 5, 6]
     const toInt = v =>
       Array.isArray(v) || Number.isNaN(v) ? v : Number.parseInt(v, 10)
@@ -42,13 +42,8 @@ export const globals = {
 
     return {}
   },
-  style(value) {
-    return { style: value }
-  },
-  theme(value, marpit) {
-    if (!marpit.themeSet.has(value)) return {}
-    return { theme: value }
-  },
+  style: v => ({ style: v }),
+  theme: (v, marpit) => (marpit.themeSet.has(v) ? { theme: v } : {}),
 }
 
 /**
@@ -77,38 +72,16 @@ export const globals = {
  * @prop {Directive} paginate Show page number on the slide if you set `true`.
  */
 export const locals = {
-  backgroundColor(value) {
-    return { backgroundColor: value }
-  },
-  backgroundImage(value) {
-    return { backgroundImage: value }
-  },
-  backgroundPosition(value) {
-    return { backgroundPosition: value }
-  },
-  backgroundRepeat(value) {
-    return { backgroundRepeat: value }
-  },
-  backgroundSize(value) {
-    return { backgroundSize: value }
-  },
-  class(value) {
-    return { class: Array.isArray(value) ? value.join(' ') : value }
-  },
-  color(value) {
-    return { color: value }
-  },
-  footer(value) {
-    return typeof value === 'string' ? { footer: value } : {}
-  },
-  header(value) {
-    return typeof value === 'string' ? { header: value } : {}
-  },
-  paginate(value) {
-    return { paginate: (value || '').toLowerCase() === 'true' }
-  },
+  backgroundColor: v => ({ backgroundColor: v }),
+  backgroundImage: v => ({ backgroundImage: v }),
+  backgroundPosition: v => ({ backgroundPosition: v }),
+  backgroundRepeat: v => ({ backgroundRepeat: v }),
+  backgroundSize: v => ({ backgroundSize: v }),
+  class: v => ({ class: Array.isArray(v) ? v.join(' ') : v }),
+  color: v => ({ color: v }),
+  footer: v => (typeof v === 'string' ? { footer: v } : {}),
+  header: v => (typeof v === 'string' ? { header: v } : {}),
+  paginate: v => ({ paginate: (v || '').toLowerCase() === 'true' }),
 }
 
-const directiveNames = [...Object.keys(globals), ...Object.keys(locals)]
-
-export default directiveNames
+export default [...Object.keys(globals), ...Object.keys(locals)]
diff --git a/src/markdown/directives/parse.js b/src/markdown/directives/parse.js
index e7d6c97b..9eb34833 100644
--- a/src/markdown/directives/parse.js
+++ b/src/markdown/directives/parse.js
@@ -1,7 +1,7 @@
 /** @module */
 import MarkdownItFrontMatter from 'markdown-it-front-matter'
 import yaml from './yaml'
-import { globals, locals } from './directives'
+import * as directives from './directives'
 
 /**
  * Parse Marpit directives and store result to the slide token meta.
@@ -44,6 +44,17 @@ function parse(md, marpit, opts = {}) {
     token.meta.marpitCommentParsed = 'directive'
   }
 
+  const filterBuiltinDirective = newProps => {
+    const ret = {}
+
+    for (const prop of Object.keys(newProps).filter(
+      p => !directives.default.includes(p)
+    ))
+      ret[prop] = newProps[prop]
+
+    return ret
+  }
+
   // Parse global directives
   md.core.ruler.after('inline', 'marpit_directives_global_parse', state => {
     if (state.inlineMode) return
@@ -55,11 +66,19 @@ function parse(md, marpit, opts = {}) {
       for (const key of Object.keys(obj)) {
         const globalKey = key.startsWith('$') ? key.slice(1) : key
 
-        if (globals[globalKey]) {
+        if (directives.globals[globalKey]) {
+          recognized = true
+          globalDirectives = {
+            ...globalDirectives,
+            ...directives.globals[globalKey](obj[key], marpit),
+          }
+        } else if (marpit.customDirectives.global[globalKey]) {
           recognized = true
           globalDirectives = {
             ...globalDirectives,
-            ...globals[globalKey](obj[key], marpit),
+            ...filterBuiltinDirective(
+              marpit.customDirectives.global[globalKey](obj[key], marpit)
+            ),
           }
         }
       }
@@ -97,9 +116,20 @@ function parse(md, marpit, opts = {}) {
       let recognized = false
 
       for (const key of Object.keys(obj)) {
-        if (locals[key]) {
+        if (directives.locals[key]) {
           recognized = true
-          cursor.local = { ...cursor.local, ...locals[key](obj[key], marpit) }
+          cursor.local = {
+            ...cursor.local,
+            ...directives.locals[key](obj[key], marpit),
+          }
+        } else if (marpit.customDirectives.local[key]) {
+          recognized = true
+          cursor.local = {
+            ...cursor.local,
+            ...filterBuiltinDirective(
+              marpit.customDirectives.local[key](obj[key], marpit)
+            ),
+          }
         }
 
         // Spot directives
@@ -107,11 +137,19 @@ function parse(md, marpit, opts = {}) {
         if (key.startsWith('_')) {
           const spotKey = key.slice(1)
 
-          if (locals[spotKey]) {
+          if (directives.locals[spotKey]) {
+            recognized = true
+            cursor.spot = {
+              ...cursor.spot,
+              ...directives.locals[spotKey](obj[key], marpit),
+            }
+          } else if (marpit.customDirectives.local[spotKey]) {
             recognized = true
             cursor.spot = {
               ...cursor.spot,
-              ...locals[spotKey](obj[key], marpit),
+              ...filterBuiltinDirective(
+                marpit.customDirectives.local[spotKey](obj[key], marpit)
+              ),
             }
           }
         }
diff --git a/src/marpit.js b/src/marpit.js
index 519ddbe8..78f868a5 100644
--- a/src/marpit.js
+++ b/src/marpit.js
@@ -76,7 +76,7 @@ class Marpit {
      * value of options after creating instance.
      *
      * @member {Object} options
-     * @memberOf Marpit#
+     * @memberOf Marpit
      * @readonly
      */
     Object.defineProperty(this, 'options', {
@@ -84,6 +84,23 @@ class Marpit {
       value: Object.freeze({ ...defaultOptions, ...opts }),
     })
 
+    /**
+     * Definitions of the custom directive.
+     *
+     * It has the assignable `global` and `local` object. They have consisted of
+     * the directive name as a key, and parser function as a value. The parser
+     * should return the validated object for updating meta of markdown-it
+     * token.
+     *
+     * @member {Object} customDirectives
+     * @memberOf Marpit
+     * @readonly
+     */
+    Object.defineProperty(this, 'customDirectives', {
+      value: Object.seal({ global: {}, local: {} }),
+    })
+
+    // Internal members
     Object.defineProperties(this, {
       containers: { value: [...wrapArray(this.options.container)] },
       slideContainers: { value: [...wrapArray(this.options.slideContainer)] },
@@ -121,7 +138,7 @@ class Marpit {
       .use(marpitStyleParse, this)
       .use(marpitSlide)
       .use(marpitParseDirectives, this, { looseYAML })
-      .use(marpitApplyDirectives)
+      .use(marpitApplyDirectives, this)
       .use(marpitHeaderAndFooter)
       .use(marpitHeadingDivider, this)
       .use(marpitSlideContainer, this.slideContainers)
diff --git a/test/markdown/background_image.js b/test/markdown/background_image.js
index e94a20ee..a4fea1cf 100644
--- a/test/markdown/background_image.js
+++ b/test/markdown/background_image.js
@@ -12,6 +12,7 @@ const splitBackgroundKeywords = ['left', 'right']
 
 describe('Marpit background image plugin', () => {
   const marpitStub = svg => ({
+    customDirectives: { global: {}, local: {} },
     lastGlobalDirectives: {},
     themeSet: { getThemeProp: () => 100 },
     options: { inlineSVG: svg },
@@ -22,7 +23,7 @@ describe('Marpit background image plugin', () => {
       .use(comment)
       .use(slide)
       .use(parseDirectives, marpitStub(svg))
-      .use(applyDirectives)
+      .use(applyDirectives, marpitStub(svg))
       .use(inlineSVG, marpitStub(svg))
       .use(parseImage, { filters })
       .use(backgroundImage)
diff --git a/test/markdown/collect.js b/test/markdown/collect.js
index 027d4ad1..1ccffb41 100644
--- a/test/markdown/collect.js
+++ b/test/markdown/collect.js
@@ -13,6 +13,7 @@ describe('Marpit collect plugin', () => {
 
   const marpitStub = (svg = false) => ({
     themeSet,
+    customDirectives: { global: {}, local: {} },
     lastGlobalDirectives: {},
     options: { inlineSVG: svg },
   })
@@ -21,8 +22,11 @@ describe('Marpit collect plugin', () => {
     new MarkdownIt('commonmark')
       .use(comment)
       .use(slide)
-      .use(parseDirectives, { themeSet: marpitInstance.themeSet })
-      .use(applyDirectives)
+      .use(parseDirectives, {
+        customDirectives: marpitInstance.customDirectives,
+        themeSet: marpitInstance.themeSet,
+      })
+      .use(applyDirectives, marpitInstance)
       .use(collect, marpitInstance)
       .use(inlineSVG, marpitInstance)
 
diff --git a/test/markdown/directives/apply.js b/test/markdown/directives/apply.js
index 9a6c8f32..a206e196 100644
--- a/test/markdown/directives/apply.js
+++ b/test/markdown/directives/apply.js
@@ -10,12 +10,15 @@ describe('Marpit directives apply plugin', () => {
   const themeSetStub = new Map()
   themeSetStub.set('test_theme', true)
 
-  const md = (...args) =>
-    new MarkdownIt('commonmark')
+  const md = (...args) => {
+    const customDirectives = { global: {}, local: {} }
+
+    return new MarkdownIt('commonmark')
       .use(comment)
       .use(slide)
-      .use(parseDirectives, { themeSet: themeSetStub })
-      .use(applyDirectives, ...args)
+      .use(parseDirectives, { customDirectives, themeSet: themeSetStub })
+      .use(applyDirectives, { customDirectives }, ...args)
+  }
 
   const mdForTest = (...args) =>
     md(...args).use(mdInstance => {
@@ -86,19 +89,6 @@ describe('Marpit directives apply plugin', () => {
     })
   })
 
-  context('with includeInternal option as true', () => {
-    const opts = { includeInternal: true }
-
-    it('applies together with unknown (internal) directive', () => {
-      const $ = cheerio.load(mdForTest(opts).render(basicDirs))
-      const section = $('section').first()
-      const style = toObjStyle(section.attr('style'))
-
-      expect(section.attr('data-unknown-dir')).toBe('directive')
-      expect(style['--unknown-dir']).toBe('directive')
-    })
-  })
-
   describe('Local directives', () => {
     describe('Background image', () => {
       const bgDirs = dedent`
diff --git a/test/markdown/directives/parse.js b/test/markdown/directives/parse.js
index a011c04c..6a365f5c 100644
--- a/test/markdown/directives/parse.js
+++ b/test/markdown/directives/parse.js
@@ -6,7 +6,10 @@ import slide from '../../../src/markdown/slide'
 
 describe('Marpit directives parse plugin', () => {
   const themeSetStub = new Map()
-  const marpitStub = { themeSet: themeSetStub }
+  const marpitStub = {
+    customDirectives: { global: {}, local: {} },
+    themeSet: themeSetStub,
+  }
   themeSetStub.set('test_theme', true)
 
   const md = (...args) =>
diff --git a/test/markdown/header_and_footer.js b/test/markdown/header_and_footer.js
index 8ac595ac..c8aa0433 100644
--- a/test/markdown/header_and_footer.js
+++ b/test/markdown/header_and_footer.js
@@ -12,6 +12,7 @@ describe('Marpit header and footer plugin', () => {
 
   const marpitStub = (props = {}) => ({
     themeSet,
+    customDirectives: { global: {}, local: {} },
     lastGlobalDirectives: {},
     ...props,
   })
@@ -20,8 +21,11 @@ describe('Marpit header and footer plugin', () => {
     new MarkdownIt('commonmark')
       .use(comment)
       .use(slide)
-      .use(parseDirectives, { themeSet: marpitInstance.themeSet })
-      .use(applyDirectives)
+      .use(parseDirectives, {
+        customDirectives: marpitInstance.customDirectives,
+        themeSet: marpitInstance.themeSet,
+      })
+      .use(applyDirectives, marpitInstance)
       .use(headerAndFooter)
 
   describe('Header local directive', () => {
diff --git a/test/markdown/heading_divider.js b/test/markdown/heading_divider.js
index 8d775c5c..14c84b59 100644
--- a/test/markdown/heading_divider.js
+++ b/test/markdown/heading_divider.js
@@ -7,6 +7,7 @@ import slide from '../../src/markdown/slide'
 
 describe('Marpit heading divider plugin', () => {
   const marpitStub = headingDividerOption => ({
+    customDirectives: { global: {}, local: {} },
     options: { headingDivider: headingDividerOption },
   })
 
@@ -106,7 +107,12 @@ describe('Marpit heading divider plugin', () => {
   })
 
   describe('Global directive', () => {
-    const md = (marpitInstance = { options: {} }) =>
+    const md = (
+      marpitInstance = {
+        customDirectives: { global: {}, local: {} },
+        options: {},
+      }
+    ) =>
       new MarkdownIt('commonmark')
         .use(comment)
         .use(pluginMd => pluginMd.core.ruler.push('marpit_slide', () => {}))
diff --git a/test/markdown/inline_svg.js b/test/markdown/inline_svg.js
index 489af998..91924e65 100644
--- a/test/markdown/inline_svg.js
+++ b/test/markdown/inline_svg.js
@@ -8,6 +8,7 @@ import { Theme, ThemeSet } from '../../src/index'
 
 describe('Marpit inline SVG plugin', () => {
   const marpitStub = (props = {}) => ({
+    customDirectives: { global: {}, local: {} },
     themeSet: new ThemeSet(),
     lastGlobalDirectives: {},
     options: { inlineSVG: true },
@@ -17,8 +18,11 @@ describe('Marpit inline SVG plugin', () => {
   const md = (marpitInstance = marpitStub()) =>
     new MarkdownIt('commonmark')
       .use(slide)
-      .use(parseDirectives, { themeSet: marpitInstance.themeSet })
-      .use(applyDirectives)
+      .use(parseDirectives, {
+        customDirectives: marpitInstance.customDirectives,
+        themeSet: marpitInstance.themeSet,
+      })
+      .use(applyDirectives, marpitInstance)
       .use(inlineSVG, marpitInstance)
 
   const render = (markdownIt, text, inline = false) => {
diff --git a/test/markdown/style/assign.js b/test/markdown/style/assign.js
index 22229342..d1ab1cb3 100644
--- a/test/markdown/style/assign.js
+++ b/test/markdown/style/assign.js
@@ -10,6 +10,7 @@ import styleParse from '../../../src/markdown/style/parse'
 
 describe('Marpit style assign plugin', () => {
   const marpitStub = (...opts) => ({
+    customDirectives: { global: {}, local: {} },
     options: { inlineStyle: true },
     themeSet: new Map(),
     ...opts,
@@ -109,7 +110,7 @@ describe('Marpit style assign plugin', () => {
         .use(comment)
         .use(slide)
         .use(parseDirectives, marpit)
-        .use(applyDirectives)
+        .use(applyDirectives, marpit)
         .use(styleAssign, marpit)
 
     it('assigns parsed style global directive to Marpit lastStyles property', () => {
@@ -131,7 +132,7 @@ describe('Marpit style assign plugin', () => {
         .use(styleParse, marpit)
         .use(slide)
         .use(parseDirectives, marpit)
-        .use(applyDirectives)
+        .use(applyDirectives, marpit)
         .use(styleAssign, marpit)
 
     it('assigns inline styles prior to directive style', () => {
diff --git a/test/markdown/sweep.js b/test/markdown/sweep.js
index 4ee462c2..baaaa07f 100644
--- a/test/markdown/sweep.js
+++ b/test/markdown/sweep.js
@@ -18,6 +18,7 @@ describe('Marpit sweep plugin', () => {
 
   it('sweeps blank paragraph made by background image plugin', () => {
     const marpitStub = {
+      customDirectives: { global: {}, local: {} },
       themeSet: new Map(),
       options: { inlineSVG: false },
     }
@@ -25,7 +26,7 @@ describe('Marpit sweep plugin', () => {
     const markdown = md({ breaks: true })
       .use(slide)
       .use(parseDirectives, marpitStub)
-      .use(applyDirectives)
+      .use(applyDirectives, marpitStub)
       .use(inlineSVG, marpitStub)
       .use(parseImage)
       .use(backgroundImage)
diff --git a/test/marpit.js b/test/marpit.js
index 9d8596ce..b5f1201a 100644
--- a/test/marpit.js
+++ b/test/marpit.js
@@ -11,24 +11,70 @@ describe('Marpit', () => {
   describe('#constructor', () => {
     const instance = new Marpit()
 
-    it('has default options', () => {
-      expect(instance.options.container.tag).toBe('div')
-      expect(instance.options.container.class).toBe('marpit')
-      expect(instance.options.backgroundSyntax).toBe(true)
-      expect(instance.options.markdown).toBe('commonmark')
-      expect(instance.options.printable).toBe(true)
-      expect(instance.options.slideContainer).toBe(false)
-      expect(instance.options.inlineSVG).toBe(false)
+    describe('options member', () => {
+      it('has default options', () => {
+        expect(instance.options.container.tag).toBe('div')
+        expect(instance.options.container.class).toBe('marpit')
+        expect(instance.options.backgroundSyntax).toBe(true)
+        expect(instance.options.markdown).toBe('commonmark')
+        expect(instance.options.printable).toBe(true)
+        expect(instance.options.slideContainer).toBe(false)
+        expect(instance.options.inlineSVG).toBe(false)
+      })
+
+      it('marks as immutable', () => {
+        expect(() => {
+          instance.options = { updated: true }
+        }).toThrow(TypeError)
+
+        expect(() => {
+          instance.options.printable = false
+        }).toThrow(TypeError)
+      })
     })
 
-    it('marks options property as immutable', () => {
-      expect(() => {
-        instance.options = { updated: true }
-      }).toThrow(TypeError)
+    describe('customDirectives member', () => {
+      it('has sealed', () => {
+        expect(Object.isSealed(instance.customDirectives)).toBe(true)
+
+        expect(() => {
+          delete instance.customDirectives.global
+        }).toThrow(TypeError)
+
+        expect(() => {
+          instance.customDirectives.spot = {}
+        }).toThrow(TypeError)
+      })
+
+      it('is assignable parser function and apply to rendered token', () => {
+        const marpit = new Marpit({ container: undefined })
+
+        expect(() => {
+          marpit.customDirectives.global.marp = v => ({ marp: `test ${v}` })
+        }).not.toThrowError()
 
-      expect(() => {
-        instance.options.printable = false
-      }).toThrow(TypeError)
+        const [token] = marpit.markdown.parse('<!-- marp: ok -->')
+        expect(token.meta.marpitDirectives).toStrictEqual({ marp: 'test ok' })
+      })
+
+      it('does not overload built-in directive parser', () => {
+        const marpit = new Marpit({ container: undefined })
+        marpit.customDirectives.local.class = () => ({ class: '!' })
+
+        const [token] = marpit.markdown.parse('<!-- class: ok -->')
+        expect(token.meta.marpitDirectives).toStrictEqual({ class: 'ok' })
+      })
+
+      it('cannot assign built-in directive as meta', () => {
+        const marpit = new Marpit({ container: undefined })
+        marpit.customDirectives.local.test = v => ({ test: v, class: v })
+
+        const [first, , , second] = marpit.markdown.parse(
+          '<!-- test: local -->\n***\n<!-- _test: spot -->'
+        )
+        expect(first.meta.marpitDirectives).toStrictEqual({ test: 'local' })
+        expect(second.meta.marpitDirectives).toStrictEqual({ test: 'spot' })
+      })
     })
 
     it('has themeSet property', () => {