diff --git a/encode_test.go b/encode_test.go
index 54225e9..a2d6950 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -6,20 +6,6 @@ import (
 	"github.com/goccy/go-json"
 )
 
-func assertErr(t *testing.T, err error) {
-	t.Helper()
-	if err != nil {
-		t.Fatalf("%+v", err)
-	}
-}
-
-func assertEq(t *testing.T, msg string, exp interface{}, act interface{}) {
-	t.Helper()
-	if exp != act {
-		t.Fatalf("failed to encode %s. exp=[%v] but act=[%v]", msg, exp, act)
-	}
-}
-
 func Test_Encoder(t *testing.T) {
 	t.Run("int", func(t *testing.T) {
 		bytes, err := json.Marshal(-10)
diff --git a/helper_test.go b/helper_test.go
new file mode 100644
index 0000000..26c0f12
--- /dev/null
+++ b/helper_test.go
@@ -0,0 +1,17 @@
+package json_test
+
+import "testing"
+
+func assertErr(t *testing.T, err error) {
+	t.Helper()
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+}
+
+func assertEq(t *testing.T, msg string, exp interface{}, act interface{}) {
+	t.Helper()
+	if exp != act {
+		t.Fatalf("failed to test for %s. exp=[%v] but act=[%v]", msg, exp, act)
+	}
+}