diff --git a/filter/feature.go b/filter/feature.go
index 325f5f6..0be5376 100644
--- a/filter/feature.go
+++ b/filter/feature.go
@@ -76,7 +76,7 @@ func (f *FeaturesFilter) add(fs Features) {
 }
 
 // Match returns true if a filter matches given features.
-func (f FeaturesFilter) Match(fs Features) bool {
+func (f *FeaturesFilter) Match(fs Features) bool {
 	n := &f.root
 	for _, v := range fs {
 		if k := n.has(v); k >= 0 {
@@ -92,7 +92,7 @@ func (f FeaturesFilter) Match(fs Features) bool {
 }
 
 // String implements string interface.
-func (f FeaturesFilter) String() string {
+func (f *FeaturesFilter) String() string {
 	var buf strings.Builder
 	filterString(&buf, 0, &f.root)
 	return buf.String()
diff --git a/filter/filter_test.go b/filter/filter_test.go
index 560c59b..d696d9d 100644
--- a/filter/filter_test.go
+++ b/filter/filter_test.go
@@ -156,10 +156,10 @@ func Benchmark_TokenFilter_WordFilter(b *testing.B) {
 	b.Run("specific token filter", func(b *testing.B) {
 		tokens := tnz.Tokenize(input)
 		words := []string{"人魚", "南", "の"}
-		filter := filter.NewWordFilter(words)
+		fl := filter.NewWordFilter(words)
 		b.ResetTimer()
 		for i := 0; i < b.N; i++ {
-			filter.Keep(&tokens)
+			fl.Keep(&tokens)
 		}
 	})
 }
diff --git a/filter/pos_test.go b/filter/pos_test.go
index 7c68d78..6b1d89e 100644
--- a/filter/pos_test.go
+++ b/filter/pos_test.go
@@ -76,10 +76,10 @@ func TestPOSFilter_Match(t *testing.T) {
 
 	for _, v := range testdata {
 		t.Run(v.title, func(t *testing.T) {
-			filter := filter.NewPOSFilter(v.featuresList...)
+			fl := filter.NewPOSFilter(v.featuresList...)
 			var got []string
 			for _, token := range tokens {
-				if filter.Match(token.POS()) {
+				if fl.Match(token.POS()) {
 					got = append(got, token.Surface)
 				}
 			}
@@ -126,9 +126,9 @@ func TestPOSFilter_Keep(t *testing.T) {
 	for _, v := range testdata {
 		t.Run(v.title, func(t *testing.T) {
 			tokens := tnz.Tokenize(input)
-			filter := filter.NewPOSFilter(v.featuresList...)
+			fl := filter.NewPOSFilter(v.featuresList...)
 			var got []string
-			filter.Keep(&tokens)
+			fl.Keep(&tokens)
 			for _, token := range tokens {
 				got = append(got, token.Surface)
 			}
@@ -139,8 +139,8 @@ func TestPOSFilter_Keep(t *testing.T) {
 	}
 
 	t.Run("empty input test", func(t *testing.T) {
-		filter := filter.NewPOSFilter(nil)
-		filter.Keep(nil)
+		fl := filter.NewPOSFilter(nil)
+		fl.Keep(nil)
 	})
 }
 
@@ -182,9 +182,9 @@ func TestPOSFilter_Drop(t *testing.T) {
 	for _, v := range testdata {
 		t.Run(v.title, func(t *testing.T) {
 			tokens := tnz.Tokenize(input)
-			filter := filter.NewPOSFilter(v.featuresList...)
+			fl := filter.NewPOSFilter(v.featuresList...)
 			var got []string
-			filter.Drop(&tokens)
+			fl.Drop(&tokens)
 			for _, token := range tokens {
 				got = append(got, token.Surface)
 			}
@@ -195,8 +195,8 @@ func TestPOSFilter_Drop(t *testing.T) {
 	}
 
 	t.Run("empty input test", func(t *testing.T) {
-		filter := filter.NewPOSFilter(nil)
-		filter.Drop(nil)
+		fl := filter.NewPOSFilter(nil)
+		fl.Drop(nil)
 	})
 }
 
diff --git a/filter/word_test.go b/filter/word_test.go
index 58fc6ed..4b9ccba 100644
--- a/filter/word_test.go
+++ b/filter/word_test.go
@@ -44,10 +44,10 @@ func TestWordFilter_Match(t *testing.T) {
 
 	for _, v := range testdata {
 		t.Run(v.title, func(t *testing.T) {
-			filter := filter.NewWordFilter(v.wordList)
+			fl := filter.NewWordFilter(v.wordList)
 			var got []string
 			for _, token := range tokens {
-				if filter.Match(token.Surface) {
+				if fl.Match(token.Surface) {
 					got = append(got, token.Surface)
 				}
 			}
@@ -92,9 +92,9 @@ func TestWordFilter_Keep(t *testing.T) {
 	for _, v := range testdata {
 		t.Run(v.title, func(t *testing.T) {
 			tokens := tnz.Tokenize(input)
-			filter := filter.NewWordFilter(v.wordList)
+			fl := filter.NewWordFilter(v.wordList)
 			var got []string
-			filter.Keep(&tokens)
+			fl.Keep(&tokens)
 			for _, token := range tokens {
 				got = append(got, token.Surface)
 			}
@@ -105,8 +105,8 @@ func TestWordFilter_Keep(t *testing.T) {
 	}
 
 	t.Run("empty input test", func(t *testing.T) {
-		filter := filter.NewWordFilter(nil)
-		filter.Keep(nil)
+		fl := filter.NewWordFilter(nil)
+		fl.Keep(nil)
 	})
 }
 
@@ -146,9 +146,9 @@ func TestWordFilter_Drop(t *testing.T) {
 	for _, v := range testdata {
 		t.Run(v.title, func(t *testing.T) {
 			tokens := tnz.Tokenize(input)
-			filter := filter.NewWordFilter(v.wordList)
+			fl := filter.NewWordFilter(v.wordList)
 			var got []string
-			filter.Drop(&tokens)
+			fl.Drop(&tokens)
 			for _, token := range tokens {
 				got = append(got, token.Surface)
 			}
@@ -159,8 +159,8 @@ func TestWordFilter_Drop(t *testing.T) {
 	}
 
 	t.Run("empty input test", func(t *testing.T) {
-		filter := filter.NewWordFilter(nil)
-		filter.Drop(nil)
+		fl := filter.NewWordFilter(nil)
+		fl.Drop(nil)
 	})
 }