diff --git a/README.md b/README.md
index 460ff12..3951fde 100644
--- a/README.md
+++ b/README.md
@@ -36,3 +36,6 @@ comment: ${{ github.event.comment.body }}
 merge_method: 'merge'
 mergers: 'comma separeted github usernames. every user is allowed if not specified'
 ```
+
+## Note
+**Setting Branch protection rules is recommended to avoid unexpected merge of pull requests.**
diff --git a/main.go b/main.go
index 3227a1e..3a7d20a 100644
--- a/main.go
+++ b/main.go
@@ -5,6 +5,7 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"regexp"
 	"text/template"
 	"time"
 
@@ -42,7 +43,7 @@ func main() {
 	defer f()
 	client := newGHClient(e.GithubToken)
 	if err := client.merge(ctx, e.Owner, e.Repo, e.PRNumber, e.MergeMethod); err != nil {
-		if serr := client.sendMsg(ctx, e.Owner, e.Repo, e.PRNumber, err.Error()); serr != nil {
+		if serr := client.sendMsg(ctx, e.Owner, e.Repo, e.PRNumber, errMsg(err)); serr != nil {
 			log.Fatalf("failed to send message: %v original: %v", serr, err)
 		}
 		log.Fatal(err.Error())
@@ -155,3 +156,20 @@ Labels:
 {{- end -}}
 {{- end -}}
 `))
+
+var (
+	needApproveRegexp = regexp.MustCompile("At least ([0-9]+) approving review is required by reviewers with write access")
+)
+
+// errMsg returns error message to post from error.
+// Especially handing error from github. go-github does not have error type for some cases.
+func errMsg(err error) string {
+	if err == nil {
+		return "Succeeded!"
+	}
+	ss := needApproveRegexp.FindStringSubmatch(err.Error())
+	if len(ss) == 2 {
+		return fmt.Sprintf("Need %s approving review", ss[1])
+	}
+	return err.Error()
+}
diff --git a/main_test.go b/main_test.go
index 419114e..841a5c6 100644
--- a/main_test.go
+++ b/main_test.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/google/go-github/github"
@@ -145,3 +146,41 @@ func Test_validateEnv(t *testing.T) {
 		})
 	}
 }
+
+func Test_errMsg(t *testing.T) {
+	type args struct {
+		err error
+	}
+	tests := []struct {
+		name string
+		args args
+		want string
+	}{
+		{
+			name: "ok",
+			args: args{},
+			want: "Succeeded!",
+		},
+		{
+			name: "need approval",
+			args: args{
+				err: errors.New("failed to merge pull request: PUT https://api.github.com/repos/abema/github-actions-merger/pulls/1/merge: 405 At least 2 approving review is required by reviewers with write access. []"),
+			},
+			want: "Need 2 approving review",
+		},
+		{
+			name: "internal server error",
+			args: args{
+				err: errors.New("internal server error"),
+			},
+			want: "internal server error",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := errMsg(tt.args.err); got != tt.want {
+				t.Errorf("errMsg() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}