diff --git a/kadai1/tanaka0325/imgconv/args.go b/kadai1/tanaka0325/imgconv/args.go
index c5cd6f6..4212901 100644
--- a/kadai1/tanaka0325/imgconv/args.go
+++ b/kadai1/tanaka0325/imgconv/args.go
@@ -10,6 +10,7 @@ func (args Args) uniq() []string {
 	for _, v := range args {
 		if !m[v] {
 			m[v] = true
+
 			u = append(u, v)
 		}
 	}
diff --git a/kadai1/tanaka0325/imgconv/cmd/imgconv/main.go b/kadai1/tanaka0325/imgconv/cmd/imgconv/main.go
index 10a5a56..875aefa 100644
--- a/kadai1/tanaka0325/imgconv/cmd/imgconv/main.go
+++ b/kadai1/tanaka0325/imgconv/cmd/imgconv/main.go
@@ -12,8 +12,8 @@ var options imgconv.Options
 var args imgconv.Args
 
 func init() {
-	options.From = flag.String("f", "jpg", "file extention before convert")
-	options.To = flag.String("t", "png", "file extention after convert")
+	options.From = flag.String("f", "jpg", "file extension before convert")
+	options.To = flag.String("t", "png", "file extension after convert")
 	options.DryRun = flag.Bool("n", false, "dry run")
 	flag.Parse()
 
diff --git a/kadai1/tanaka0325/imgconv/cnv_image.go b/kadai1/tanaka0325/imgconv/cnv_image.go
new file mode 100644
index 0000000..869a7d9
--- /dev/null
+++ b/kadai1/tanaka0325/imgconv/cnv_image.go
@@ -0,0 +1,81 @@
+package imgconv
+
+import (
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"io"
+
+	"golang.org/x/image/bmp"
+	"golang.org/x/image/tiff"
+)
+
+type Decoder interface {
+	Decode(io.Reader) (image.Image, error)
+}
+
+type Encoder interface {
+	Encode(io.Writer, image.Image) error
+}
+
+type DecodeEncoder interface {
+	Decoder
+	Encoder
+}
+
+type CnvImage struct{}
+
+// CnvImagePng is type for png format.
+type CnvImagePNG CnvImage
+
+func (ip CnvImagePNG) Decode(r io.Reader) (image.Image, error) { return png.Decode(r) }
+
+func (ip CnvImagePNG) Encode(w io.Writer, i image.Image) error { return png.Encode(w, i) }
+
+// CnvImageJPEG is type for jpeg format.
+type CnvImageJPEG CnvImage
+
+func (ip CnvImageJPEG) Decode(r io.Reader) (image.Image, error) { return jpeg.Decode(r) }
+
+func (ip CnvImageJPEG) Encode(w io.Writer, i image.Image) error { return jpeg.Encode(w, i, nil) }
+
+// CnvImageGIF is type for gif format.
+type CnvImageGIF CnvImage
+
+func (ip CnvImageGIF) Decode(r io.Reader) (image.Image, error) { return gif.Decode(r) }
+
+func (ip CnvImageGIF) Encode(w io.Writer, i image.Image) error {
+	return gif.Encode(w, i, &gif.Options{NumColors: 256})
+}
+
+// CnvImageBMP is type for bmp format.
+type CnvImageBMP CnvImage
+
+func (ip CnvImageBMP) Decode(r io.Reader) (image.Image, error) { return bmp.Decode(r) }
+
+func (ip CnvImageBMP) Encode(w io.Writer, i image.Image) error { return bmp.Encode(w, i) }
+
+// CnvImageTIFF is type for tiff format.
+type CnvImageTIFF CnvImage
+
+func (ip CnvImageTIFF) Decode(r io.Reader) (image.Image, error) { return tiff.Decode(r) }
+
+func (ip CnvImageTIFF) Encode(w io.Writer, i image.Image) error { return tiff.Encode(w, i, nil) }
+
+func newCnvImage(ext string) DecodeEncoder {
+	switch ext {
+	case "png":
+		return &CnvImagePNG{}
+	case "jpg", "jpeg":
+		return &CnvImageJPEG{}
+	case "gif":
+		return &CnvImageGIF{}
+	case "bmp":
+		return &CnvImageBMP{}
+	case "tiff", "tif":
+		return &CnvImageTIFF{}
+	}
+
+	return nil
+}
diff --git a/kadai1/tanaka0325/imgconv/conv_image.go b/kadai1/tanaka0325/imgconv/conv_image.go
deleted file mode 100644
index cb92ef3..0000000
--- a/kadai1/tanaka0325/imgconv/conv_image.go
+++ /dev/null
@@ -1,82 +0,0 @@
-package imgconv
-
-import (
-	"fmt"
-	"image"
-	"image/gif"
-	"image/jpeg"
-	"image/png"
-	"io"
-	"os"
-
-	"golang.org/x/image/bmp"
-	"golang.org/x/image/tiff"
-)
-
-// convImage is type included infomation to convert file format.
-type convImage struct {
-	filename string
-	fromExt  string
-	toExt    string
-	image    image.Image
-}
-
-func (i *convImage) decode() error {
-	r, err := os.Open(i.filename + "." + i.fromExt)
-	if err != nil {
-		return err
-	}
-	defer r.Close()
-
-	img, err := decodeHelper(r, i.fromExt)
-	if err != nil {
-		return fmt.Errorf("decode error: %w", err)
-	}
-
-	i.image = img
-	return nil
-}
-
-func decodeHelper(r io.Reader, ext string) (image.Image, error) {
-	switch ext {
-	case "png":
-		return png.Decode(r)
-	case "jpg", "jpeg":
-		return jpeg.Decode(r)
-	case "gif":
-		return gif.Decode(r)
-	case "bmp":
-		return bmp.Decode(r)
-	case "tiff", "tif":
-		return tiff.Decode(r)
-	}
-	return nil, fmt.Errorf("%s is not allowed", ext)
-}
-
-func (i *convImage) encode() error {
-	w, err := os.Create(i.filename + "." + i.toExt)
-	if err != nil {
-		return err
-	}
-	defer func() error {
-		if err := w.Close(); err != nil {
-			return err
-		}
-		return nil
-	}()
-
-	switch i.toExt {
-	case "png":
-		return png.Encode(w, i.image)
-	case "jpg", "jpeg":
-		return jpeg.Encode(w, i.image, nil)
-	case "gif":
-		return gif.Encode(w, i.image, nil)
-	case "bmp":
-		return gif.Encode(w, i.image, nil)
-	case "tiff", "tif":
-		return gif.Encode(w, i.image, nil)
-	}
-
-	return fmt.Errorf("cannot encode image")
-}
diff --git a/kadai1/tanaka0325/imgconv/go.mod.back b/kadai1/tanaka0325/imgconv/go.mod.back
deleted file mode 100644
index ad64074..0000000
--- a/kadai1/tanaka0325/imgconv/go.mod.back
+++ /dev/null
@@ -1,5 +0,0 @@
-module github.com/gopherdojo/dojo8/kadai1/tanaka0325/imgconv
-
-go 1.14
-
-require golang.org/x/image v0.0.0-20200618115811-c13761719519
diff --git a/kadai1/tanaka0325/imgconv/imgconv.go b/kadai1/tanaka0325/imgconv/imgconv.go
index 29db8da..e80c35c 100644
--- a/kadai1/tanaka0325/imgconv/imgconv.go
+++ b/kadai1/tanaka0325/imgconv/imgconv.go
@@ -9,47 +9,85 @@ import (
 )
 
 var allowedExts = []string{"png", "jpg", "jpeg", "gif", "bmp", "tiff", "tif"}
+var fromExt string
+var toExt string
 
-// Run is to convert image file format
+// Run is to convert image file format.
 func Run(options Options, args Args) error {
-	// validator
+	// // validator
 	if err := options.validate(allowedExts); err != nil {
 		return err
 	}
 
+	fromExt = *options.From
+	toExt = *options.To
+
 	// get target image flepaths from args
-	paths, err := getTargetFilePaths(args, *options.From)
+	paths, err := getTargetFilePaths(args, fromExt)
 	if err != nil {
 		return err
 	}
 
 	// convert
-	imgs, err := createConvImages(paths, *options.From, *options.To)
-	if err != nil {
-		return err
-	}
-	for _, img := range imgs {
-		if err := img.decode(); err != nil {
-			return err
-		}
+	for _, path := range paths {
+		filename := strings.Replace(path, "."+fromExt, "", -1)
+		// "))
+		f := newCnvImage(fromExt)
+		t := newCnvImage(toExt)
 
 		if *options.DryRun {
-			fmt.Println(img.filename+"."+img.fromExt, "=>", img.filename+"."+img.toExt)
-		} else {
-			if err := img.encode(); err != nil {
-				return err
-			}
+			fmt.Printf("%s.%s => %s.%s \n", filename, fromExt, filename, toExt)
+		} else if err := convert(f, t, filename); err != nil {
+			return err
 		}
 	}
 
 	return nil
 }
 
+func convert(d Decoder, e Encoder, filename string) (err error) {
+	// open file
+	r, err := os.Open(filename + "." + fromExt)
+	if err != nil {
+		return
+	}
+	defer r.Close()
+
+	// decode
+	img, err := d.Decode(r)
+	if err != nil {
+		return
+	}
+
+	// create file
+	w, err := os.Create(filename + "." + toExt)
+	if err != nil {
+		return err
+	}
+
+	defer func() {
+		err = w.Close()
+	}()
+
+	// encode
+	if err := e.Encode(w, img); err != nil {
+		return err
+	}
+
+	return
+}
+
 func getTargetFilePaths(args Args, from string) ([]string, error) {
 	uns := args.uniq()
-	paths := []string{}
 
+	paths := []string{}
 	for _, n := range uns {
+		if ok, err := isDir(n); err != nil {
+			return nil, err
+		} else if !ok {
+			return nil, fmt.Errorf("%s is not a directory", n)
+		}
+
 		if err := filepath.Walk(n, func(path string, info os.FileInfo, err error) error {
 			if filepath.Ext(path) == "."+from {
 				paths = append(paths, path)
@@ -63,16 +101,16 @@ func getTargetFilePaths(args Args, from string) ([]string, error) {
 	return paths, nil
 }
 
-func createConvImages(paths []string, from, to string) ([]convImage, error) {
-	images := []convImage{}
-	for _, p := range paths {
-		i := convImage{
-			filename: strings.Replace(p, "."+from, "", 1),
-			fromExt:  strings.ToLower(from),
-			toExt:    strings.ToLower(to),
-		}
-		images = append(images, i)
+func isDir(path string) (bool, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return false, err
+	}
+
+	fi, err := f.Stat()
+	if err != nil {
+		return false, err
 	}
 
-	return images, nil
+	return fi.IsDir(), nil
 }
diff --git a/kadai1/tanaka0325/imgconv/options.go b/kadai1/tanaka0325/imgconv/options.go
index 3a31e15..1d91f04 100644
--- a/kadai1/tanaka0325/imgconv/options.go
+++ b/kadai1/tanaka0325/imgconv/options.go
@@ -13,13 +13,14 @@ type Options struct {
 	DryRun *bool
 }
 
-func (opt Options) validate(allow_list []string) error {
+func (opt Options) validate(allowList []string) error {
 	to := strings.ToLower(*opt.To)
 	from := strings.ToLower(*opt.From)
 	targetExts := []string{to, from}
+
 	for _, e := range targetExts {
-		if err := include(allow_list, e); err != nil {
-			return fmt.Errorf("%w. ext is only allowd in %s", err, allow_list)
+		if err := include(allowList, e); err != nil {
+			return fmt.Errorf("%w. ext is only allowd in %s", err, allowList)
 		}
 	}