diff --git a/cmd/sum_test.go b/cmd/sum_test.go
new file mode 100644
index 0000000..3873a1a
--- /dev/null
+++ b/cmd/sum_test.go
@@ -0,0 +1,36 @@
+package cmd
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestSum(t *testing.T) {
+	cases := []struct {
+		command string
+		want    string
+	}{
+		{command: "sum -- -1 2", want: "1\n"},
+		{command: "sum --norm -- -1 2", want: "3\n"},
+	}
+
+	for _, c := range cases {
+		buf := new(bytes.Buffer)
+		cmd, err := NewRootCmd()
+		if err != nil {
+			t.Errorf("failed to create root cmd: %s", err)
+		}
+		cmd.SetOut(buf)
+		cmdArgs := strings.Split(c.command, " ")
+		cmd.SetArgs(cmdArgs)
+		if err := cmd.Execute(); err != nil {
+			t.Errorf("failed to execute cmd: %s", err)
+		}
+
+		get := buf.String()
+		if c.want != get {
+			t.Errorf("unexpected response: want:%q, get:%q", c.want, get)
+		}
+	}
+}