diff --git a/kadai1/.gitignore b/kadai1/.gitignore
new file mode 100644
index 00000000..808d8dca
--- /dev/null
+++ b/kadai1/.gitignore
@@ -0,0 +1,2 @@
+image-converter
+images/**
\ No newline at end of file
diff --git a/kadai1/README.md b/kadai1/README.md
new file mode 100644
index 00000000..b5cf9506
--- /dev/null
+++ b/kadai1/README.md
@@ -0,0 +1,44 @@
+# 画像変換コマンド
+
+## 次の仕様を満たすコマンドを作ってください
+
+- ディレクトリを指定する
+- 指定したディレクトリ以下の JPG ファイルを PNG に変換(デフォルト)
+- ディレクトリ以下は再帰的に処理する
+- 変換前と変換後の画像形式を指定できる(オプション)
+
+## 以下を満たすように開発してください
+
+- main パッケージと分離する
+- 自作パッケージと標準パッケージと準標準パッケージのみ使う
+- ユーザ定義型を作ってみる
+- GoDoc を生成してみる
+- Go Modules を使ってみる
+
+## 使い方
+
+###　ビルドして使う場合
+
+- ビルド
+
+```
+go build -o image-converter
+```
+
+- 例
+
+```
+image-converter -d sample -f .png -t .jpg
+```
+
+### ビルドしない場合
+
+```
+go run main.go -d [ディレクトリ] -f [変換前の拡張子]　-t [変換後の拡張子]
+```
+
+- 例
+
+```
+go run main.go -d sample -f .png -t .jpg
+```
diff --git a/kadai1/cli/cli.go b/kadai1/cli/cli.go
new file mode 100644
index 00000000..52f519ee
--- /dev/null
+++ b/kadai1/cli/cli.go
@@ -0,0 +1,96 @@
+package cli
+
+import (
+	"flag"
+	"fmt"
+
+	"io"
+	"io/fs"
+	"path/filepath"
+
+	"github.com/kynefuk/gopherdojo-studyroom/kadai1/converter"
+)
+
+// Exit codes are values that represents an exit code
+const (
+	ExitOk  = 0
+	ExitErr = 1
+)
+
+// CLI is a struct of cli
+type CLI struct {
+	OutStream io.Writer
+	ErrStream io.Writer
+}
+
+// Run is a main func of CLI
+func (c *CLI) Run(args []string) int {
+	var (
+		targetDir string
+		fromExt   string
+		toExt     string
+	)
+
+	flag.CommandLine.SetOutput(c.ErrStream)
+	flag.Usage = func() {
+		fmt.Fprintf(c.ErrStream, helpText)
+	}
+
+	flag.StringVar(&targetDir, "d", "", "specify target directory")
+	flag.StringVar(&fromExt, "f", converter.ExtJPG, "specify \"fromExt\"")
+	flag.StringVar(&toExt, "t", converter.ExtPNG, "specify \"toExt\"")
+
+	flag.Parse()
+
+	if ok := converter.IsConvertible(fromExt); !ok {
+		fmt.Printf("fromExt format is not convertible: %v\n", fromExt)
+		return ExitErr
+	}
+
+	if ok := converter.IsConvertible(toExt); !ok {
+		fmt.Printf("toExt format is not convertible: %v\n", toExt)
+		return ExitErr
+	}
+
+	// walking directory to collect target image files
+	var targetFiles []string
+	err := filepath.WalkDir(targetDir, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			fmt.Printf("prevent panic by handling failure accessing a path %q, error: %v\n", path, err)
+			return err
+		}
+
+		if ext := filepath.Ext(path); ext == fromExt {
+			targetFiles = append(targetFiles, path)
+			return nil
+		}
+		return nil
+	})
+
+	if err != nil {
+		fmt.Printf("failed to walk dir: %v\n", err)
+		return ExitErr
+	}
+
+	// converting
+	for _, f := range targetFiles {
+		con := converter.Converter{FromExt: fromExt, ToExt: toExt, TargetFilePath: f}
+
+		if err := con.Convert(); err != nil {
+			fmt.Printf("failed to convert: %v\n", err)
+			continue
+		}
+	}
+	return ExitOk
+}
+
+var helpText = `Usage: image-convert [options...]
+image-convert
+Options:
+-d
+	specify target dir in which images will be converted
+-f
+	specify image ext which convert from
+-t
+	specify image ext which convert to
+`
diff --git a/kadai1/converter/converter.go b/kadai1/converter/converter.go
new file mode 100644
index 00000000..80d645bf
--- /dev/null
+++ b/kadai1/converter/converter.go
@@ -0,0 +1,92 @@
+package converter
+
+import (
+	"fmt"
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"os"
+	"strings"
+)
+
+// Ext codes represents convertible file ext
+const (
+	ExtJPG  = ".jpg"
+	ExtJPEG = ".jpeg"
+	ExtPNG  = ".png"
+	ExtGIF  = ".gif"
+)
+
+// IsConvertible check that specified ext is convertible
+func IsConvertible(ext string) bool {
+	switch ext {
+	case ExtJPG, ExtJPEG, ExtPNG, ExtGIF:
+		return true
+	default:
+		return false
+	}
+}
+
+// Converter is ...
+type Converter struct {
+	FromExt        string
+	ToExt          string
+	TargetFilePath string
+}
+
+// Convert is a main func of image convert
+func (c *Converter) Convert() error {
+	var err error
+
+	reader, err := os.Open(c.TargetFilePath)
+	if err != nil {
+		fmt.Printf("failed to open file: %v. error: %v", c.TargetFilePath, err)
+		return err
+	}
+	defer reader.Close()
+
+	// decode
+	var img image.Image
+
+	switch c.FromExt {
+	case ExtJPG, ExtJPEG:
+		img, err = jpeg.Decode(reader)
+	case ExtPNG:
+		img, err = png.Decode(reader)
+	case ExtGIF:
+		img, err = gif.Decode(reader)
+	}
+
+	if err != nil {
+		fmt.Printf("failed to decode: %v\n", err)
+		return err
+	}
+
+	// create dist file
+	fileName := strings.Replace(c.TargetFilePath, c.FromExt, c.ToExt, 1)
+	dist, err := os.Create(fileName)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := dist.Close(); err != nil {
+			fmt.Printf("failed to close file: %v\n", err)
+		}
+	}()
+
+	// encode
+	switch c.ToExt {
+	case ExtJPG, ExtJPEG:
+		err = jpeg.Encode(dist, img, &jpeg.Options{})
+	case ExtPNG:
+		err = png.Encode(dist, img)
+	case ExtGIF:
+		err = gif.Encode(dist, img, &gif.Options{})
+	}
+
+	if err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/kadai1/go.mod b/kadai1/go.mod
new file mode 100644
index 00000000..0e6642be
--- /dev/null
+++ b/kadai1/go.mod
@@ -0,0 +1,3 @@
+module github.com/kynefuk/gopherdojo-studyroom/kadai1
+
+go 1.17
diff --git a/kadai1/main.go b/kadai1/main.go
new file mode 100644
index 00000000..12f9b267
--- /dev/null
+++ b/kadai1/main.go
@@ -0,0 +1,15 @@
+package main
+
+import (
+	"os"
+
+	"github.com/kynefuk/gopherdojo-studyroom/kadai1/cli"
+)
+
+func main() {
+	cli := cli.CLI{
+		OutStream: os.Stdout,
+		ErrStream: os.Stderr,
+	}
+	os.Exit(cli.Run(os.Args))
+}