diff --git a/internal/common/testutil/testutil.go b/internal/common/testutil/testutil.go
new file mode 100644
index 0000000..946bdd5
--- /dev/null
+++ b/internal/common/testutil/testutil.go
@@ -0,0 +1,45 @@
+package testutil
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+func NoError(t *testing.T, err error) {
+	t.Helper()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func Error(t *testing.T, err error) {
+	t.Helper()
+	if err == nil {
+		t.Fatal(err)
+	}
+}
+
+func IsError(t *testing.T, actual, expected error) {
+	t.Helper()
+	if !errors.Is(actual, expected) {
+		t.Fatalf("not equal. actual: %v, expected: %v", actual, expected)
+	}
+}
+
+func ErrorContains(t *testing.T, err error, errStr string) {
+	t.Helper()
+	if err == nil {
+		t.Fatal("error should occur")
+	}
+	if !strings.Contains(err.Error(), errStr) {
+		t.Fatalf("error does not contain '%s'. err: %v", errStr, err)
+	}
+}
+
+func Equal[T comparable](t *testing.T, actual, expected T) {
+	t.Helper()
+	if actual != expected {
+		t.Fatalf("not equal. actual: %v, expected: %v", actual, expected)
+	}
+}
diff --git a/internal/stream/decoding/string_test.go b/internal/stream/decoding/string_test.go
new file mode 100644
index 0000000..e3c7ea5
--- /dev/null
+++ b/internal/stream/decoding/string_test.go
@@ -0,0 +1,100 @@
+package decoding
+
+import (
+	"bytes"
+	"errors"
+	"github.com/shamaton/msgpack/v2/def"
+	"github.com/shamaton/msgpack/v2/internal/common"
+	"github.com/shamaton/msgpack/v2/internal/common/testutil"
+	"io"
+	"math"
+	"reflect"
+	"testing"
+)
+
+var errReaderErr = errors.New("reader error")
+
+type errReader struct{}
+
+func (errReader) Read(p []byte) (n int, err error) {
+	return 0, errReaderErr
+}
+
+func Test_stringByteLength(t *testing.T) {
+	testcases := []struct {
+		name     string
+		code     byte
+		length   int
+		expected int
+		errSkip  bool
+	}{
+		{
+			name:     "FixStr",
+			code:     def.FixStr + 1,
+			expected: 1,
+			errSkip:  true,
+		},
+		{
+			name:     "Str8",
+			code:     def.Str8,
+			length:   1,
+			expected: math.MaxUint8,
+		},
+		{
+			name:     "Str16",
+			code:     def.Str16,
+			length:   2,
+			expected: math.MaxUint16,
+		},
+		{
+			name:     "Str32",
+			code:     def.Str32,
+			length:   4,
+			expected: math.MaxUint32,
+		},
+		{
+			name:     "Nil",
+			code:     def.Nil,
+			expected: 0,
+			errSkip:  true,
+		},
+	}
+
+	for _, tc := range testcases {
+		t.Run(tc.name+"", func(t *testing.T) {
+			t.Run("ng", func(t *testing.T) {
+				if tc.errSkip {
+					t.Log("this testcase is skipped by skip flag")
+					return
+				}
+				d := decoder{
+					r:   &errReader{},
+					buf: common.GetBuffer(),
+				}
+				defer common.PutBuffer(d.buf)
+				_, err := d.stringByteLength(tc.code, reflect.String)
+				testutil.IsError(t, err, errReaderErr)
+			})
+			t.Run("ok", func(t *testing.T) {
+				data := make([]byte, tc.length)
+				for i := range data {
+					data[i] = 0xff
+				}
+
+				d := decoder{
+					r:   bytes.NewReader(data),
+					buf: common.GetBuffer(),
+				}
+				defer common.PutBuffer(d.buf)
+				v, err := d.stringByteLength(tc.code, reflect.String)
+				testutil.NoError(t, err)
+				testutil.Equal(t, v, tc.expected)
+
+				p := make([]byte, 1)
+				n, err := d.r.Read(p)
+				testutil.IsError(t, err, io.EOF)
+				testutil.Equal(t, n, 0)
+			})
+		})
+	}
+}