diff --git a/cmd/sum.go b/cmd/sum.go
new file mode 100644
index 0000000..ea548e1
--- /dev/null
+++ b/cmd/sum.go
@@ -0,0 +1,20 @@
+package cmd
+
+import (
+	"fmt"
+	"github.com/mpppk/cli-template/lib"
+	"github.com/spf13/cobra"
+)
+
+var sumCmd = &cobra.Command{
+	Use:   "sum",
+	Short: "print sum of arguments",
+	Long: ``,
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Println(lib.SumFromString(args))
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(sumCmd)
+}
diff --git a/lib/sum.go b/lib/sum.go
new file mode 100644
index 0000000..9cb6954
--- /dev/null
+++ b/lib/sum.go
@@ -0,0 +1,17 @@
+package lib
+
+func Sum(numbers []int) (sum int) {
+	for _, number := range numbers {
+		sum += number
+	}
+	return
+}
+
+func SumFromString(stringNumbers []string) (sum int, err error) {
+	numbers, err := ConvertStringSliceToIntSlice(stringNumbers)
+	if err != nil {
+		return 0, err
+	}
+	return Sum(numbers), nil
+}
+
diff --git a/lib/sum_test.go b/lib/sum_test.go
new file mode 100644
index 0000000..f72b76a
--- /dev/null
+++ b/lib/sum_test.go
@@ -0,0 +1,70 @@
+package lib
+
+import "testing"
+
+func TestSum(t *testing.T) {
+	type args struct {
+		numbers []int
+	}
+	tests := []struct {
+		name    string
+		args    args
+		wantSum int
+	}{
+		{
+			name: "return sum of numbers",
+			args: args{
+				numbers: []int{1,2,3},
+			},
+			wantSum: 6,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if gotSum := Sum(tt.args.numbers); gotSum != tt.wantSum {
+				t.Errorf("Sum() = %v, want %v", gotSum, tt.wantSum)
+			}
+		})
+	}
+}
+
+func TestSumFromString(t *testing.T) {
+	type args struct {
+		stringNumbers []string
+	}
+	tests := []struct {
+		name    string
+		args    args
+		wantSum int
+		wantErr bool
+	}{
+		{
+			name: "return sum of numbers",
+			args: args{
+				stringNumbers: []string{"1", "2", "3"},
+			},
+			wantSum: 6,
+			wantErr: false,
+		},
+		{
+			name: "will be error if args includes not number string",
+			args: args{
+				stringNumbers: []string{"1", "2", "a"},
+			},
+			wantSum: 0,
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotSum, err := SumFromString(tt.args.stringNumbers)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("SumFromString() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if gotSum != tt.wantSum {
+				t.Errorf("SumFromString() = %v, want %v", gotSum, tt.wantSum)
+			}
+		})
+	}
+}
diff --git a/lib/util.go b/lib/util.go
new file mode 100644
index 0000000..a37ffdd
--- /dev/null
+++ b/lib/util.go
@@ -0,0 +1,29 @@
+package lib
+
+import (
+	"fmt"
+	"github.com/pkg/errors"
+	"os"
+	"strconv"
+)
+
+func ConvertStringSliceToIntSlice(stringSlice []string) (intSlice []int, err error) {
+	for _, s := range stringSlice {
+		num, err := strconv.Atoi(s)
+		if err != nil {
+			return nil, errors.Wrap(err, "failed to convert string slice to int slice")
+		}
+		intSlice = append(intSlice, num)
+	}
+	return
+}
+
+func EPrintlnIFErrExist(err error, msg string) (bool, error) {
+	if err != nil {
+		if _, err := fmt.Fprintln(os.Stderr, msg); err != nil {
+			return true, err
+		}
+		return true, nil
+	}
+	return false, nil
+}
diff --git a/lib/util_test.go b/lib/util_test.go
new file mode 100644
index 0000000..2c0bb1a
--- /dev/null
+++ b/lib/util_test.go
@@ -0,0 +1,47 @@
+package lib
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestConvertStringSliceToIntSlice(t *testing.T) {
+	type args struct {
+		stringSlice []string
+	}
+	tests := []struct {
+		name         string
+		args         args
+		wantIntSlice []int
+		wantErr      bool
+	}{
+		{
+			name: "can convert string slice to int slice",
+			args: args{
+				stringSlice: []string{"1", "2", "3"},
+			},
+			wantIntSlice: []int{1, 2, 3},
+			wantErr: false,
+		},
+		{
+			name: "will be error if string can not be convert to number",
+			args: args{
+				stringSlice: []string{"1", "2", "a"},
+			},
+			wantIntSlice: nil,
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotIntSlice, err := ConvertStringSliceToIntSlice(tt.args.stringSlice)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ConvertStringSliceToIntSlice() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(gotIntSlice, tt.wantIntSlice) {
+				t.Errorf("ConvertStringSliceToIntSlice() = %v, want %v", gotIntSlice, tt.wantIntSlice)
+			}
+		})
+	}
+}