diff --git a/data/class/helper/SC_Helper_TaxRule.php b/data/class/helper/SC_Helper_TaxRule.php
index 206d6d34cd..44ca75158b 100644
--- a/data/class/helper/SC_Helper_TaxRule.php
+++ b/data/class/helper/SC_Helper_TaxRule.php
@@ -75,7 +75,7 @@ public static function sfTax($price, $product_id = 0, $product_class_id = 0, $pr
      * @param int $discount_total 値引額合計
      * @return array{8?:array{total:int,tax:int}, 10?:array{total:int,tax:int}}
      */
-    public static function getTaxPerTaxRate(array $arrTaxableTotal, int $discount_total = 0): array
+    public static function getTaxPerTaxRate(array $arrTaxableTotal, $discount_total = 0)
     {
         $arrDefaultTaxRule = static::getTaxRule();
 
@@ -104,7 +104,7 @@ public static function getTaxPerTaxRate(array $arrTaxableTotal, int $discount_to
      * @param int $discount_total 値引額合計
      * @return string (<税率>%対象: <値引後税込合計>円 内消費税: <値引後税額>円)
      */
-    public static function getTaxDetail($arrTaxableTotal, $discount_total = 0): string
+    public static function getTaxDetail($arrTaxableTotal, $discount_total = 0)
     {
         $arrTaxPerTaxRate = static::getTaxPerTaxRate($arrTaxableTotal, $discount_total);
         $result = '';
diff --git a/tests/class/helper/SC_Helper_TaxRule/SC_Helper_TaxRule_getTaxDetailTest.php b/tests/class/helper/SC_Helper_TaxRule/SC_Helper_TaxRule_getTaxDetailTest.php
new file mode 100644
index 0000000000..1f67e12aa7
--- /dev/null
+++ b/tests/class/helper/SC_Helper_TaxRule/SC_Helper_TaxRule_getTaxDetailTest.php
@@ -0,0 +1,159 @@
+<?php
+require __DIR__.'/SC_Helper_TaxRule_TestBase.php';
+
+class SC_Helper_TaxRule_getDetailTest extends SC_Helper_TaxRule_TestBase
+{
+    private $taxs = [];
+    protected function setUp()
+    {
+        parent::setUp();
+    }
+
+    protected function tearDown()
+    {
+        parent::tearDown();
+    }
+
+    public function testGetTaxPerTaxRateWithRound()
+    {
+        $this->setUpTaxRule([
+            [
+                'tax_rule_id' => 1004,
+                'apply_date' => '2019-10-01 00:00:00',
+                'tax_rate' => '10',
+                'calc_rule' => '1',
+                'product_id' => '0',
+                'product_class_id' => '0',
+                'del_flg' => '0',
+                'member_id' => 1,
+                'create_date' => '2000-01-01 00:00:00',
+                'update_date' => '2000-01-01 00:00:00',
+            ],
+        ]);
+
+        $arrTaxableTotal = [
+            10 => 724431,
+            8 => 65756,
+        ];
+        $discount_total = 7159;
+
+        $actual = SC_Helper_TaxRule_Ex::getTaxPerTaxRate($arrTaxableTotal, $discount_total);
+        self::assertSame(
+            [
+                8 => [
+                    'total' => 65160,
+                    'tax' => 4827
+                ],
+                10 => [
+                    'total' => 717868,
+                    'tax' => 65261
+                ]
+            ],
+            $actual
+        );
+
+        self::assertSame(
+            '(8%対象: 65,160円 内消費税: 4,827円)'.PHP_EOL.
+                '(10%対象: 717,868円 内消費税: 65,261円)'.PHP_EOL,
+            SC_Helper_TaxRule_Ex::getTaxDetail($arrTaxableTotal, $discount_total)
+        );
+    }
+
+    /**
+     * @runInSeparateProcess
+     * @preserveGlobalState disabled
+     */
+    public function testGetTaxPerTaxRateWithFloor()
+    {
+        self::markTestSkipped('Skip this test because @runInSeparateProcess does not work properly');
+
+        $this->setUpTaxRule([
+            [
+                'tax_rule_id' => 1004,
+                'apply_date' => '2019-10-01 00:00:00',
+                'tax_rate' => '10',
+                'calc_rule' => '2', // floor
+                'product_id' => '0',
+                'product_class_id' => '0',
+                'del_flg' => '0',
+                'member_id' => 1,
+                'create_date' => '2000-01-01 00:00:00',
+                'update_date' => '2000-01-01 00:00:00',
+            ],
+        ]);
+
+        $arrTaxableTotal = [
+            10 => 724431,
+            8 => 65756,
+        ];
+        $discount_total = 7159;
+
+        $actual = SC_Helper_TaxRule_Ex::getTaxPerTaxRate($arrTaxableTotal, $discount_total);
+        self::assertSame(
+            [
+                8 => [
+                    'total' => 65160,
+                    'tax' => 4826
+                ],
+                10 => [
+                    'total' => 717867,
+                    'tax' => 65260
+                ]
+            ],
+            $actual
+        );
+    }
+
+    /**
+     * @runInSeparateProcess
+     * @preserveGlobalState disabled
+     */
+    public function testGetTaxPerTaxRateWithCeil()
+    {
+        self::markTestSkipped('Skip this test because @runInSeparateProcess does not work properly');
+
+        $this->setUpTaxRule([
+            [
+                'tax_rule_id' => 1004,
+                'apply_date' => '2019-10-01 00:00:00',
+                'tax_rate' => '10',
+                'calc_rule' => '3', // ceil
+                'product_id' => '0',
+                'product_class_id' => '0',
+                'del_flg' => '0',
+                'member_id' => 1,
+                'create_date' => '2000-01-01 00:00:00',
+                'update_date' => '2000-01-01 00:00:00',
+            ],
+        ]);
+
+        $arrTaxableTotal = [
+            10 => 724431,
+            8 => 65756,
+        ];
+        $discount_total = 7159;
+
+        $actual = SC_Helper_TaxRule_Ex::getTaxPerTaxRate($arrTaxableTotal, $discount_total);
+        self::assertSame(
+            [
+                8 => [
+                    'total' => 65161,
+                    'tax' => 4827
+                ],
+                10 => [
+                    'total' => 717868,
+                    'tax' => 65261
+                ]
+            ],
+            $actual
+        );
+    }
+
+    protected function setUpTaxRule(array $taxs = [])
+    {
+        $this->objQuery->delete('dtb_tax_rule');
+        foreach ($taxs as $key => $item) {
+            $this->objQuery->insert('dtb_tax_rule', $item);
+        }
+    }
+}