diff --git a/kadai2/po3rin/.gitkeep b/kadai2/po3rin/.gitkeep
new file mode 100644
index 0000000..e69de29
diff --git a/kadai2/po3rin/Makefile b/kadai2/po3rin/Makefile
new file mode 100644
index 0000000..a665d36
--- /dev/null
+++ b/kadai2/po3rin/Makefile
@@ -0,0 +1,28 @@
+GOCMD=go
+GOBUILD=$(GOCMD) build
+GOCLEAN=$(GOCMD) clean
+GOTEST=$(GOCMD) test
+GOGET=$(GOCMD) get
+GODOC=$(GOCMD) doc
+COVERAGEFILE=coverage.out
+BINARY_NAME=chext
+BINARY_UNIX=$(BINARY_NAME)_unix
+
+all: test build
+build:
+	$(GOBUILD) -o $(BINARY_NAME)
+test:
+	$(GOTEST) -v ./extension/
+coverage:
+	$(GOTEST) -coverprofile=$(COVERAGEFILE) ./extension/
+doc:
+	$(GODOC) ./extension
+clean:
+	$(GOCLEAN)
+	rm -f $(BINARY_NAME)
+	rm -f $(BINARY_UNIX)
+	rm -f $(COVERAGEFILE)
+build-linux:
+	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 $(GOBUILD) -o $(BINARY_UNIX) -v
+docker-build:
+	docker run --rm -it -v "$(GOPATH)":/go -w /go/src/bitbucket.org/rsohlich/makepost golang:latest go build -o "$(BINARY_UNIX)" -v
\ No newline at end of file
diff --git a/kadai2/po3rin/README.md b/kadai2/po3rin/README.md
new file mode 100644
index 0000000..d50194e
--- /dev/null
+++ b/kadai2/po3rin/README.md
@@ -0,0 +1,54 @@
+# GopherDojo Task #1 By po3rin
+
+This CLI convert extension of images. .png, .jpeg, .jpg and .gif is only supported.
+
+# Feature
+
+Traveling the directory structure recursively.
+
+## Quick Start
+
+There are test-images in /images directory.
+default behavior is to convert jpg to png in /images directory.
+
+```
+$ make
+$ ./chext
+```
+
+## Flag
+
+You can set arguments.
+
+|  Flag  |  Description  | Default |
+| ---- | ---- | --- |
+|  -f  |  What images converted from  | jpg |
+|  -t  |  What images converted to  | png |
+|  -d  |  Designate directory that has images | . |
+
+following is example command with flags. This command convert jpg to gif in images/sub directory.
+
+```
+./chext -f jpg -t gif -d images/sub
+```
+
+## Doc
+
+following command show GoDoc
+
+```
+make doc
+```
+
+## 感想と課題
+
+全パターンは Table Driven Tests を実装した。convert する全6パターンが非常に見やすくなった。しかし、mapにfor文を適用すると、書いた順番で繰り返されない仕様にハマった。for文で順番を担保するにはどうしたらいいのだろう。。
+
+カバレッジは80%いくが他の20%はエラー処理。
+エラー処理もカバーする必要がある？
+
+サブテストとテーブル駆動テストのパターンを採用した場合、テストヘルパーを使っても呼び出し行が変わらないのであまり意味がない？？そもそも使い方が間違っているかもしれない。
+
+## 宿題 io.Writer と io.Reader について
+io.Writer io.Readerはそれぞれインターフェースであり、抽象化されている。故に振る舞いを与えられているだけなのでFile Buffer等の様々なものに対して実装されている。
+ゆえに実装が使い回せるので、結果的にGo言語のシンプルさに貢献している。
diff --git a/kadai2/po3rin/extension/extension.go b/kadai2/po3rin/extension/extension.go
new file mode 100644
index 0000000..e1ac90b
--- /dev/null
+++ b/kadai2/po3rin/extension/extension.go
@@ -0,0 +1,60 @@
+// Package extension create & change extention of images.
+package extension
+
+import (
+	"fmt"
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"os"
+	"strings"
+)
+
+// Arg define comand line argument
+type Arg struct {
+	From string
+	To   string
+	Path string
+}
+
+// Convert create & convert extension of images. png jpg jpeg gif are only supported.
+func (a Arg) Convert() error {
+	if a.From != "jpg" && a.From != "png" && a.From != "gif" {
+		return fmt.Errorf("%s is not supported", a.From)
+	}
+	if a.To != "jpg" && a.To != "png" && a.To != "gif" {
+		return fmt.Errorf("%s is not supported", a.To)
+	}
+
+	file, err := os.Open(a.Path)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+	img, _, err := image.Decode(file)
+	if err != nil {
+		return err
+	}
+
+	extention := a.To
+	output := strings.Replace(a.Path, a.From, "", 1)
+	dstfile, err := os.Create(output + extention)
+	if err != nil {
+		return err
+	}
+	defer dstfile.Close()
+
+	switch extention {
+	case "jpeg", "jpg":
+		err = jpeg.Encode(dstfile, img, nil)
+	case "gif":
+		err = gif.Encode(dstfile, img, nil)
+	case "png":
+		err = png.Encode(dstfile, img)
+	}
+	if err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/kadai2/po3rin/extension/extension_test.go b/kadai2/po3rin/extension/extension_test.go
new file mode 100644
index 0000000..1b08577
--- /dev/null
+++ b/kadai2/po3rin/extension/extension_test.go
@@ -0,0 +1,49 @@
+package extension_test
+
+import (
+	"dojo2/kadai2/po3rin/extension"
+	"strconv"
+	"testing"
+)
+
+func TestArg_Convert(t *testing.T) {
+	testarg := []extension.Arg{
+		{
+			From: "jpg",
+			To:   "png",
+			Path: "../images/gopher.jpg",
+		}, {
+			From: "jpg",
+			To:   "gif",
+			Path: "../images/gopher.jpg",
+		}, {
+			From: "pndg",
+			To:   "jpg",
+			Path: "../images/gopher.png",
+		}, {
+			From: "png",
+			To:   "gif",
+			Path: "../images/gopher.png",
+		}, {
+			From: "gidf",
+			To:   "jpg",
+			Path: "../images/gopher.gif",
+		}, {
+			From: "gif",
+			To:   "png",
+			Path: "../images/gopher.gif",
+		},
+	}
+	for i, v := range testarg {
+		t.Run("test"+strconv.Itoa(i), func(t *testing.T) {
+			helperConvert(v, t)
+		})
+	}
+}
+
+func helperConvert(v extension.Arg, t *testing.T) {
+	t.Helper()
+	if err := v.Convert(); err != nil {
+		t.Errorf("failed test %#v", err)
+	}
+}
diff --git a/kadai2/po3rin/images/gopher.gif b/kadai2/po3rin/images/gopher.gif
new file mode 100644
index 0000000..723c0d9
Binary files /dev/null and b/kadai2/po3rin/images/gopher.gif differ
diff --git a/kadai2/po3rin/images/gopher.jpg b/kadai2/po3rin/images/gopher.jpg
new file mode 100644
index 0000000..0ad29f9
Binary files /dev/null and b/kadai2/po3rin/images/gopher.jpg differ
diff --git a/kadai2/po3rin/images/gopher.png b/kadai2/po3rin/images/gopher.png
new file mode 100644
index 0000000..33c2410
Binary files /dev/null and b/kadai2/po3rin/images/gopher.png differ
diff --git a/kadai2/po3rin/images/sub/gopher-sub.gif b/kadai2/po3rin/images/sub/gopher-sub.gif
new file mode 100644
index 0000000..7384514
Binary files /dev/null and b/kadai2/po3rin/images/sub/gopher-sub.gif differ
diff --git a/kadai2/po3rin/images/sub/gopher-sub.jpg b/kadai2/po3rin/images/sub/gopher-sub.jpg
new file mode 100644
index 0000000..98afdf4
Binary files /dev/null and b/kadai2/po3rin/images/sub/gopher-sub.jpg differ
diff --git a/kadai2/po3rin/images/sub/gopher-sub.png b/kadai2/po3rin/images/sub/gopher-sub.png
new file mode 100644
index 0000000..d374076
Binary files /dev/null and b/kadai2/po3rin/images/sub/gopher-sub.png differ
diff --git a/kadai2/po3rin/main.go b/kadai2/po3rin/main.go
new file mode 100644
index 0000000..93ef54d
--- /dev/null
+++ b/kadai2/po3rin/main.go
@@ -0,0 +1,32 @@
+package main
+
+import (
+	"dojo2/kadai2/po3rin/extension"
+	"flag"
+	"log"
+	"os"
+	"path/filepath"
+)
+
+func main() {
+	from := flag.String("f", "jpg", "what encode from")
+	to := flag.String("t", "png", "what encode to")
+	dir := flag.String("d", ".", "target dir")
+	flag.Parse()
+
+	err := filepath.Walk(*dir, func(path string, info os.FileInfo, err error) error {
+		if filepath.Ext(path) == "."+*from {
+			arg := extension.Arg{
+				From: *from,
+				To:   *to,
+				Path: path,
+			}
+			return arg.Convert()
+		}
+		return nil
+	})
+	if err != nil {
+		log.Fatal(err)
+		os.Exit(1)
+	}
+}