diff --git a/go.mod b/go.mod
index 30277f8..ea844d2 100644
--- a/go.mod
+++ b/go.mod
@@ -3,13 +3,9 @@ module github.com/logica0419/resigif
 go 1.22.2
 
 require (
-	github.com/stretchr/testify v1.9.0
+	github.com/youta-t/its v0.5.0
 	golang.org/x/image v0.19.0
 	golang.org/x/sync v0.8.0
 )
 
-require (
-	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/pmezard/go-difflib v1.0.0 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
-)
+require golang.org/x/mod v0.20.0 // indirect
diff --git a/go.sum b/go.sum
index c76aae4..619d4b5 100644
--- a/go.sum
+++ b/go.sum
@@ -1,14 +1,10 @@
-github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
-github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
-github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+github.com/youta-t/its v0.5.0 h1:KScN5T5OHZPcj4KdPWmoOjLSyMRbG21LPB2dijxQKQA=
+github.com/youta-t/its v0.5.0/go.mod h1:BIW52yCMszkfw3y00YtIB5I5EP10+vVJFu3cV78DqlU=
 golang.org/x/image v0.19.0 h1:D9FX4QWkLfkeqaC62SonffIIuYdOk/UE2XKUBgRIBIQ=
 golang.org/x/image v0.19.0/go.mod h1:y0zrRqlQRWQ5PXaYCOMLTW2fpsxZ8Qh9I/ohnInJEys=
+golang.org/x/mod v0.15.0 h1:SernR4v+D55NyBH2QiEQrlBAnj1ECL6AGrA5+dPaMY8=
+golang.org/x/mod v0.15.0/go.mod h1:hTbmBsO62+eylJbnUtE2MGJUyE7QWk4xUqPFrRgJ+7c=
+golang.org/x/mod v0.20.0 h1:utOm6MM3R3dnawAiJgn0y+xvuYRsm1RKM/4giyfDgV0=
+golang.org/x/mod v0.20.0/go.mod h1:hTbmBsO62+eylJbnUtE2MGJUyE7QWk4xUqPFrRgJ+7c=
 golang.org/x/sync v0.8.0 h1:3NFvSEYkUoMifnESzZl15y791HH1qU2xm6eCJU5ZPXQ=
 golang.org/x/sync v0.8.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
-gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/resigif_test.go b/resigif_test.go
index a6cec41..42508f5 100644
--- a/resigif_test.go
+++ b/resigif_test.go
@@ -10,8 +10,7 @@ import (
 
 	"github.com/logica0419/resigif"
 	"github.com/logica0419/resigif/testdata"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
+	"github.com/youta-t/its"
 	"golang.org/x/image/draw"
 )
 
@@ -21,14 +20,14 @@ func mustOpenGif(t *testing.T, name string) *gif.GIF {
 	t.Helper()
 
 	file, err := testdata.FS.Open(name)
-	require.NoError(t, err)
+	its.Nil[error]().Match(err).OrFatal(t)
 
 	defer func() {
 		_ = file.Close()
 	}()
 
 	image, err := gif.DecodeAll(file)
-	require.NoError(t, err)
+	its.Nil[error]().Match(err).OrFatal(t)
 
 	return image
 }
@@ -37,14 +36,14 @@ func mustEncodeGif(t *testing.T, name string, image *gif.GIF) {
 	t.Helper()
 
 	file, err := os.OpenFile(filepath.Clean("testdata/"+name), os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o600)
-	require.NoError(t, err)
+	its.Nil[error]().Match(err).OrFatal(t)
 
 	defer func() {
 		_ = file.Close()
 	}()
 
 	err = gif.EncodeAll(file, image)
-	require.NoError(t, err)
+	its.Nil[error]().Match(err).OrFatal(t)
 }
 
 func TestResize(t *testing.T) {
@@ -61,10 +60,10 @@ func TestResize(t *testing.T) {
 	}
 
 	tests := []struct {
-		name      string
-		args      args
-		want      string
-		assertion require.ErrorAssertionFunc
+		name       string
+		args       args
+		want       string
+		errMatcher its.Matcher[error]
 	}{
 		{
 			name: "success (mushroom 正方形、小サイズ)",
@@ -77,8 +76,8 @@ func TestResize(t *testing.T) {
 					resigif.WithAspectRatio(resigif.Ignore),
 				},
 			},
-			want:      "mushroom_resized.gif",
-			assertion: require.NoError,
+			want:       "mushroom_resized.gif",
+			errMatcher: its.Nil[error](),
 		},
 		{
 			name: "success (tooth 正方形、DisposalBackground)",
@@ -91,8 +90,8 @@ func TestResize(t *testing.T) {
 					resigif.WithParallel(1),
 				},
 			},
-			want:      "tooth_resized.gif",
-			assertion: require.NoError,
+			want:       "tooth_resized.gif",
+			errMatcher: its.Nil[error](),
 		},
 		{
 			name: "success (new_year 横長)",
@@ -105,8 +104,8 @@ func TestResize(t *testing.T) {
 					resigif.WithImageResizeFunc(resigif.FromDrawScaler(draw.BiLinear)),
 				},
 			},
-			want:      "new_year_resized.gif",
-			assertion: require.NoError,
+			want:       "new_year_resized.gif",
+			errMatcher: its.Nil[error](),
 		},
 		{
 			name: "success (miku 縦長、差分最適化)",
@@ -117,8 +116,8 @@ func TestResize(t *testing.T) {
 				height: 256,
 				opts:   nil,
 			},
-			want:      "miku_resized.gif",
-			assertion: require.NoError,
+			want:       "miku_resized.gif",
+			errMatcher: its.Nil[error](),
 		},
 		{
 			name: "success (frog 縦長、DisposalBackground + 背景色不整合)",
@@ -129,8 +128,8 @@ func TestResize(t *testing.T) {
 				height: 256,
 				opts:   nil,
 			},
-			want:      "frog_resized.gif",
-			assertion: require.NoError,
+			want:       "frog_resized.gif",
+			errMatcher: its.Nil[error](),
 		},
 		{
 			name: "success (surprised 正方形、空のGlobal Color Table)",
@@ -141,8 +140,8 @@ func TestResize(t *testing.T) {
 				height: 256,
 				opts:   nil,
 			},
-			want:      "surprised_resized.gif",
-			assertion: require.NoError,
+			want:       "surprised_resized.gif",
+			errMatcher: its.Nil[error](),
 		},
 	}
 
@@ -156,8 +155,8 @@ func TestResize(t *testing.T) {
 				mustEncodeGif(t, tt.want, got)
 			}
 
-			tt.assertion(t, err)
-			assert.Equal(t, mustOpenGif(t, tt.want), got)
+			tt.errMatcher.Match(err).OrError(t)
+			its.DeepEqual(mustOpenGif(t, tt.want)).Match(got).OrError(t)
 		})
 	}
 }