diff --git a/kadai2/akuchii/README.md b/kadai2/akuchii/README.md
new file mode 100644
index 0000000..d6d1b46
--- /dev/null
+++ b/kadai2/akuchii/README.md
@@ -0,0 +1,8 @@
+# io.Readerとio.Writerについて調べてみよう
+
+## 標準パッケージでどのように使われているか
+os.File, os.Stdin, os.Stdout, bytes.buffersなどデータの読み書きを行う処理の部分に使われている
+
+## io.Readerとio.Writerがあることでどういう利点があるのか具体例を挙げて考えてみる
+例えばデータを読み書きする処理があったときに、io.Reader, io.Writerがあることで使う側は内部実装を知らなくても使うことができる。
+内部実装を知らずに使えるので、読み書き先をDBからjson, csvなどに切り替えたい時も容易に行うことができる。
diff --git a/kadai2/akuchii/converter/converter.go b/kadai2/akuchii/converter/converter.go
new file mode 100644
index 0000000..3cd0d20
--- /dev/null
+++ b/kadai2/akuchii/converter/converter.go
@@ -0,0 +1,159 @@
+// Package converter converts image extension to target extension
+package converter
+
+import (
+	"fmt"
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+var supportedExts = map[string]bool{
+	"png":  true,
+	"jpeg": true,
+	"jpg":  true,
+	"gif":  true,
+}
+
+// Encoder encodes image
+type Encoder interface {
+	Encode(io.Writer, image.Image) error
+}
+
+// Converter converts image to other format of afterExt
+type Converter struct {
+	path     string
+	outDir   string
+	afterExt string
+	encoder  Encoder
+}
+
+// JpegEncoder encodes image to jpeg format
+type JpegEncoder struct {
+}
+
+// PngEncoder encodes image to png format
+type PngEncoder struct {
+}
+
+// GifEncoder encodes image to gif format
+type GifEncoder struct {
+}
+
+// Encode encodes to jpeg
+func (c *JpegEncoder) Encode(w io.Writer, img image.Image) error {
+	return jpeg.Encode(w, img, nil)
+}
+
+// Encode encodes to png
+func (c *PngEncoder) Encode(w io.Writer, img image.Image) error {
+	return png.Encode(w, img)
+}
+
+// Encode encodes to gif
+func (c *GifEncoder) Encode(w io.Writer, img image.Image) error {
+	return gif.Encode(w, img, nil)
+}
+
+// Encode encodes to specific format
+func (c *Converter) Encode(w io.Writer, img image.Image) error {
+	return c.encoder.Encode(w, img)
+}
+
+// Decode decodes input image
+func (c *Converter) Decode(r io.Reader) (image.Image, error) {
+	img, _, err := image.Decode(r)
+	return img, err
+}
+
+// NewConverter creates new converter instance
+func NewConverter(path, outDir, afterExt string) (*Converter, error) {
+	var encoder Encoder
+	switch afterExt {
+	case "jpg", "jpeg":
+		encoder = &JpegEncoder{}
+	case "png":
+		encoder = &PngEncoder{}
+	case "gif":
+		encoder = &GifEncoder{}
+	}
+	return &Converter{path, outDir, afterExt, encoder}, nil
+}
+
+// Execute reads file from path, convert file extension to afterExt and save it in outDir
+func (c *Converter) Execute() error {
+	if _, ok := supportedExts[c.afterExt]; !ok {
+		return fmt.Errorf("%s is not supported ext", c.afterExt)
+	}
+
+	in, err := os.Open(c.path)
+	if err != nil {
+		return err
+	}
+
+	defer in.Close()
+
+	img, err := c.Decode(in)
+	if err != nil {
+		return err
+	}
+
+	err = c.createDstDir()
+	if err != nil {
+		return err
+	}
+
+	dstPath, err := c.getDstPath()
+	if err != nil {
+		return err
+	}
+
+	out, err := os.Create(dstPath)
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+
+	err = c.Encode(out, img)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (c *Converter) getDstDir() (string, error) {
+	srcAbs, err := filepath.Abs(c.path)
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(filepath.Dir(srcAbs), c.outDir), nil
+}
+
+func (c *Converter) createDstDir() error {
+	dstDir, err := c.getDstDir()
+	if err != nil {
+		return err
+	}
+
+	if _, err := os.Stat(dstDir); os.IsNotExist(err) {
+		err = os.MkdirAll(dstDir, os.ModePerm)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (c *Converter) getDstPath() (string, error) {
+	filenameWithoutExt := filepath.Base(c.path[:len(c.path)-len(filepath.Ext(c.path))])
+	dstDir, err := c.getDstDir()
+	if err != nil {
+		return "", err
+	}
+	dstPath := filepath.Join(dstDir, filenameWithoutExt) + "." + c.afterExt
+	return dstPath, nil
+}
diff --git a/kadai2/akuchii/converter/converter_test.go b/kadai2/akuchii/converter/converter_test.go
new file mode 100644
index 0000000..2d91882
--- /dev/null
+++ b/kadai2/akuchii/converter/converter_test.go
@@ -0,0 +1,72 @@
+package converter_test
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+
+	"github.com/gopherdojo/dojo4/kadai2/akuchii/converter"
+)
+
+func TestConvert_NewConverter(t *testing.T) {
+	cases := []struct {
+		afterExt        string
+		expectedEncoder reflect.Type
+	}{
+		{"png", reflect.ValueOf(&converter.PngEncoder{}).Type()},
+		{"jpeg", reflect.ValueOf(&converter.JpegEncoder{}).Type()},
+		{"gif", reflect.ValueOf(&converter.GifEncoder{}).Type()},
+	}
+	for _, c := range cases {
+		conv, err := converter.NewConverter("", "", c.afterExt)
+		if err != nil {
+			t.Fatalf("failed to crete new converter:%s", err)
+		}
+		if reflect.ValueOf(conv.ExportEncoder()).Type() != c.expectedEncoder {
+			t.Fatalf("invalid encoder")
+		}
+	}
+}
+
+func TestConverter_Execute(t *testing.T) {
+	cases := []struct {
+		target   string
+		outDir   string
+		afterExt string
+		hasError bool
+	}{
+		{"test.png", "out", "jpeg", false},
+		{"test.jpeg", "out", "gif", false},
+		{"test.gif", "out", "png", false},
+		{"test.webp", "out", "png", true},
+		{"test.png", "out", "webp", true},
+		{"not_exist_file.png", "out", "jpeg", true},
+	}
+
+	for _, c := range cases {
+		srcPath := filepath.Join("..", "testdata", c.target)
+		filenameWithoutExt := testGetFileNameWithoutExt(t, c.target)
+		dstPath := filepath.Join("..", "testdata", c.outDir, filenameWithoutExt) + "." + c.afterExt
+		conv, err := converter.NewConverter(srcPath, c.outDir, c.afterExt)
+		if err != nil {
+			t.Fatalf("fail to create converter: %s", err)
+		}
+		err = conv.Execute()
+		if err != nil && !c.hasError {
+			t.Fatalf("fail to convert image: %s", err)
+		}
+		if _, err := os.Stat(dstPath); os.IsNotExist(err) && !c.hasError {
+			t.Fatalf("fail to generate converted image: %s", err)
+		}
+		if !c.hasError {
+			os.Remove(dstPath)
+		}
+	}
+}
+
+func testGetFileNameWithoutExt(t *testing.T, target string) string {
+	t.Helper()
+
+	return target[:len(target)-len(filepath.Ext(target))]
+}
diff --git a/kadai2/akuchii/converter/export_test.go b/kadai2/akuchii/converter/export_test.go
new file mode 100644
index 0000000..5d715f3
--- /dev/null
+++ b/kadai2/akuchii/converter/export_test.go
@@ -0,0 +1,5 @@
+package converter
+
+func (c *Converter) ExportEncoder() Encoder {
+	return c.encoder
+}
diff --git a/kadai2/akuchii/main.go b/kadai2/akuchii/main.go
new file mode 100644
index 0000000..75ac147
--- /dev/null
+++ b/kadai2/akuchii/main.go
@@ -0,0 +1,88 @@
+package main
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/gopherdojo/dojo4/kadai2/akuchii/converter"
+)
+
+// extension before convert
+var beforeExt string
+
+// extension after convert
+var afterExt string
+
+// source directory
+var srcDir string
+
+// destination directory
+var dstDir string
+
+// exit code
+const (
+	ExitCodeOK    int = iota // 0
+	ExitCodeError            // 1
+)
+
+func init() {
+	flag.StringVar(&beforeExt, "b", "jpeg", "extension before convert")
+	flag.StringVar(&afterExt, "a", "png", "extension before convert")
+	flag.StringVar(&srcDir, "s", "", "source directory to convert files")
+	flag.StringVar(&dstDir, "d", "out", "destination directory to convert files")
+}
+
+func main() {
+	flag.Parse()
+
+	err := validateArgs()
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(ExitCodeError)
+	}
+
+	fmt.Printf("start to convert ext before: %s, after: %s\n", beforeExt, afterExt)
+	err = filepath.Walk(srcDir, func(path string, info os.FileInfo, err error) error {
+		if filepath.Ext(path) == "."+beforeExt {
+			conv, err := converter.NewConverter(path, dstDir, afterExt)
+			if err != nil {
+				fmt.Println(err)
+				os.Exit(ExitCodeError)
+			}
+
+			err = conv.Execute()
+			if err != nil {
+				fmt.Println(err)
+				os.Exit(ExitCodeError)
+			}
+			fmt.Printf("%s is converted to %s\n", path, afterExt)
+		}
+		return nil
+	})
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(ExitCodeError)
+	}
+	fmt.Println("convert is finished")
+	os.Exit(ExitCodeOK)
+}
+
+// validateArgs validates input arguments
+func validateArgs() error {
+	if beforeExt == "" || afterExt == "" || srcDir == "" || dstDir == "" {
+		flag.PrintDefaults()
+		return errors.New("empty arg is not allowed")
+	}
+
+	if _, err := os.Stat(srcDir); os.IsNotExist(err) {
+		return err
+	}
+
+	if beforeExt == afterExt {
+		return fmt.Errorf("before and after ext is same: %s", beforeExt)
+	}
+	return nil
+}
diff --git a/kadai2/akuchii/testdata/out/.gitkeep b/kadai2/akuchii/testdata/out/.gitkeep
new file mode 100644
index 0000000..e69de29
diff --git a/kadai2/akuchii/testdata/test.gif b/kadai2/akuchii/testdata/test.gif
new file mode 100644
index 0000000..ac37093
Binary files /dev/null and b/kadai2/akuchii/testdata/test.gif differ
diff --git a/kadai2/akuchii/testdata/test.jpeg b/kadai2/akuchii/testdata/test.jpeg
new file mode 100644
index 0000000..97f9e84
Binary files /dev/null and b/kadai2/akuchii/testdata/test.jpeg differ
diff --git a/kadai2/akuchii/testdata/test.png b/kadai2/akuchii/testdata/test.png
new file mode 100644
index 0000000..fd588d9
Binary files /dev/null and b/kadai2/akuchii/testdata/test.png differ
diff --git a/kadai2/akuchii/testdata/test.webp b/kadai2/akuchii/testdata/test.webp
new file mode 100644
index 0000000..7175cb1
Binary files /dev/null and b/kadai2/akuchii/testdata/test.webp differ