diff --git a/cli.go b/cli.go
new file mode 100644
index 0000000..6a6b5fd
--- /dev/null
+++ b/cli.go
@@ -0,0 +1,62 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+)
+
+type CmdArgs struct {
+	Level string
+	Args  []string
+}
+
+func ParseArgs() (*CmdArgs, error) {
+	opts := CmdArgs{}
+
+	flag.Usage = flagHelpMessage
+	flag.StringVar(&opts.Level, "level", "normal", "slot difficulty. [easy|normal|hard]")
+	flag.Parse()
+	opts.Args = flag.Args()
+
+	if err := opts.Validate(); err != nil {
+		return nil, err
+	}
+
+	return &opts, nil
+}
+
+func flagHelpMessage() {
+	cmd := os.Args[0]
+	fmt.Fprintln(os.Stderr, fmt.Sprintf("%s changes file permissions with a slot", cmd))
+	fmt.Fprintln(os.Stderr, "")
+	fmt.Fprintln(os.Stderr, "Usage:")
+	fmt.Fprintln(os.Stderr, fmt.Sprintf("  %s [OPTIONS] [files...]", cmd))
+	fmt.Fprintln(os.Stderr, "")
+	fmt.Fprintln(os.Stderr, "Examples:")
+	fmt.Fprintln(os.Stderr, fmt.Sprintf("  %s sample.txt", cmd))
+	fmt.Fprintln(os.Stderr, "")
+	fmt.Fprintln(os.Stderr, "Options:")
+
+	flag.PrintDefaults()
+}
+
+func (c *CmdArgs) Validate() error {
+	if len(c.Args) < 1 {
+		return fmt.Errorf("Must need files")
+	}
+
+	_, ok := slotIntervalTime[c.Level]
+	if !ok {
+		return fmt.Errorf("-level must be 'eash' or 'normal' or 'hard'.")
+	}
+
+	for _, file := range c.Args {
+		_, err := os.Stat(file)
+		if os.IsNotExist(err) {
+			return fmt.Errorf("%s file doesn't exist.", file)
+		}
+	}
+
+	return nil
+}
diff --git a/cli_test.go b/cli_test.go
new file mode 100644
index 0000000..948bfaf
--- /dev/null
+++ b/cli_test.go
@@ -0,0 +1,70 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCmdArgsValidate(t *testing.T) {
+	tests := []struct {
+		desc    string
+		args    CmdArgs
+		wantErr bool
+	}{
+		{
+			desc: "ok",
+			args: CmdArgs{
+				Level: "normal",
+				Args:  []string{"LICENSE"},
+			},
+			wantErr: false,
+		},
+		{
+			desc: "ok: level is hard",
+			args: CmdArgs{
+				Level: "hard",
+				Args:  []string{"LICENSE"},
+			},
+			wantErr: false,
+		},
+		{
+			desc: "ng: arguments count is 0",
+			args: CmdArgs{
+				Level: "normal",
+				Args:  []string{},
+			},
+			wantErr: true,
+		},
+		{
+			desc: "ng: illegal level string",
+			args: CmdArgs{
+				Level: "aiueo",
+				Args:  []string{"LICENSE"},
+			},
+			wantErr: true,
+		},
+		{
+			desc: "ng: file doesn't exist",
+			args: CmdArgs{
+				Level: "aiueo",
+				Args:  []string{"LICENSE", "sushi.txt"},
+			},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			assert := assert.New(t)
+
+			err := tt.args.Validate()
+			if tt.wantErr {
+				assert.Error(err)
+				return
+			}
+
+			assert.NoError(err)
+		})
+	}
+}
diff --git a/log.go b/log.go
new file mode 100644
index 0000000..a048b6e
--- /dev/null
+++ b/log.go
@@ -0,0 +1,10 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func Err(err error) {
+	fmt.Fprintln(os.Stderr, "[ERR] "+err.Error())
+}
diff --git a/main.go b/main.go
index 0a26de4..1aecfef 100644
--- a/main.go
+++ b/main.go
@@ -1,7 +1,6 @@
 package main
 
 import (
-	"flag"
 	"fmt"
 	"os"
 	"strconv"
@@ -10,10 +9,6 @@ import (
 	termbox "github.com/nsf/termbox-go"
 )
 
-type CmdOpts struct {
-	Level string
-}
-
 var (
 	slotIntervalTime = map[string]int{
 		"easy":   200,
@@ -23,31 +18,13 @@ var (
 )
 
 func main() {
-	opts := CmdOpts{}
-	flag.Usage = flagHelpMessage
-	flag.StringVar(&opts.Level, "level", "normal", "slot difficulty. [easy|normal|hard]")
-	flag.Parse()
-	args := flag.Args()
-
-	if len(args) < 1 {
-		fmt.Fprintln(os.Stderr, "[ERR] Must need files")
+	args, err := ParseArgs()
+	if err != nil {
+		Err(err)
 		os.Exit(1)
 	}
 
-	interval, ok := slotIntervalTime[opts.Level]
-	if !ok {
-		fmt.Fprintln(os.Stderr, "[ERR] -level must be 'eash' or 'normal' or 'hard'.")
-		os.Exit(2)
-	}
-
-	for _, file := range args {
-		_, err := os.Stat(file)
-		if os.IsNotExist(err) {
-			fmt.Fprintln(os.Stderr, fmt.Sprintf("[ERR] %s file doesn't exist.", file))
-			os.Exit(3)
-		}
-	}
-
+	interval := slotIntervalTime[args.Level]
 	slot := NewSlot(0, interval)
 
 	if err := termbox.Init(); err != nil {
@@ -61,28 +38,13 @@ func main() {
 	waitKeyInput(slot)
 	termbox.Close()
 
-	changeMode(slot, args)
-}
-
-func flagHelpMessage() {
-	cmd := os.Args[0]
-	fmt.Fprintln(os.Stderr, fmt.Sprintf("%s changes file permissions with a slot", cmd))
-	fmt.Fprintln(os.Stderr, "")
-	fmt.Fprintln(os.Stderr, "Usage:")
-	fmt.Fprintln(os.Stderr, fmt.Sprintf("  %s [OPTIONS] [files...]", cmd))
-	fmt.Fprintln(os.Stderr, "")
-	fmt.Fprintln(os.Stderr, "Examples:")
-	fmt.Fprintln(os.Stderr, fmt.Sprintf("  %s sample.txt", cmd))
-	fmt.Fprintln(os.Stderr, "")
-	fmt.Fprintln(os.Stderr, "Options:")
-
-	flag.PrintDefaults()
+	changeMode(slot, args.Args)
 }
 
 func clock(s *Slot) {
 	for !s.IsFinished() {
 		s.Switch()
-		drawSlot(s)
+		DrawSlot(s)
 		time.Sleep(time.Duration(s.IntervalTime()) * time.Millisecond)
 	}
 }
diff --git a/view.go b/view.go
index cfaf4ca..2f11983 100644
--- a/view.go
+++ b/view.go
@@ -6,7 +6,7 @@ import (
 	termbox "github.com/nsf/termbox-go"
 )
 
-func drawSlot(s *Slot) {
+func DrawSlot(s *Slot) {
 	termbox.Clear(termbox.ColorDefault, termbox.ColorDefault)
 
 	idx := s.CurrentSlotIndex()