diff --git a/cmd/vartan/parse.go b/cmd/vartan/parse.go
index 2f06732..d1fc80e 100644
--- a/cmd/vartan/parse.go
+++ b/cmd/vartan/parse.go
@@ -9,6 +9,7 @@ import (
 
 	"github.com/nihei9/vartan/driver"
 	spec "github.com/nihei9/vartan/spec/grammar"
+	"github.com/nihei9/vartan/tester"
 	"github.com/spf13/cobra"
 )
 
@@ -17,9 +18,15 @@ var parseFlags = struct {
 	onlyParse  *bool
 	cst        *bool
 	disableLAC *bool
-	json       *bool
+	format     *string
 }{}
 
+const (
+	outputFormatText = "text"
+	outputFormatTree = "tree"
+	outputFormatJSON = "json"
+)
+
 func init() {
 	cmd := &cobra.Command{
 		Use:     "parse <grammar file path>",
@@ -32,7 +39,7 @@ func init() {
 	parseFlags.onlyParse = cmd.Flags().Bool("only-parse", false, "when this option is enabled, the parser performs only parse and doesn't semantic actions")
 	parseFlags.cst = cmd.Flags().Bool("cst", false, "when this option is enabled, the parser generates a CST")
 	parseFlags.disableLAC = cmd.Flags().Bool("disable-lac", false, "disable LAC (lookahead correction)")
-	parseFlags.json = cmd.Flags().Bool("json", false, "enable JSON output")
+	parseFlags.format = cmd.Flags().StringP("format", "f", "text", "output format: one of text|tree|json")
 	rootCmd.AddCommand(cmd)
 }
 
@@ -40,6 +47,11 @@ func runParse(cmd *cobra.Command, args []string) error {
 	if *parseFlags.onlyParse && *parseFlags.cst {
 		return fmt.Errorf("You cannot enable --only-parse and --cst at the same time")
 	}
+	if *parseFlags.format != outputFormatText &&
+		*parseFlags.format != outputFormatTree &&
+		*parseFlags.format != outputFormatJSON {
+		return fmt.Errorf("invalid output format: %v", *parseFlags.format)
+	}
 
 	cg, err := readCompiledGrammar(args[0])
 	if err != nil {
@@ -108,13 +120,17 @@ func runParse(cmd *cobra.Command, args []string) error {
 			tree = tb.Tree()
 		}
 		if tree != nil {
-			if *parseFlags.json {
+			switch *parseFlags.format {
+			case "tree":
+				b := tester.ConvertSyntaxTreeToTestableTree(tree).Format()
+				fmt.Fprintln(os.Stdout, string(b))
+			case "json":
 				b, err := json.Marshal(tree)
 				if err != nil {
 					return err
 				}
 				fmt.Fprintln(os.Stdout, string(b))
-			} else {
+			default:
 				driver.PrintTree(os.Stdout, tree)
 			}
 		}
diff --git a/spec/test/parser.go b/spec/test/parser.go
index 0513ee3..175c89e 100644
--- a/spec/test/parser.go
+++ b/spec/test/parser.go
@@ -57,6 +57,30 @@ func (t *Tree) path() string {
 	return fmt.Sprintf("%v.[%v]%v", t.Parent.path(), t.Offset, t.Kind)
 }
 
+func (t *Tree) Format() []byte {
+	var b bytes.Buffer
+	t.format(&b, 0)
+	return b.Bytes()
+}
+
+func (t *Tree) format(buf *bytes.Buffer, depth int) {
+	for i := 0; i < depth; i++ {
+		buf.WriteString("    ")
+	}
+	buf.WriteString("(")
+	buf.WriteString(t.Kind)
+	if len(t.Children) > 0 {
+		buf.WriteString("\n")
+		for i, c := range t.Children {
+			c.format(buf, depth+1)
+			if i < len(t.Children)-1 {
+				buf.WriteString("\n")
+			}
+		}
+	}
+	buf.WriteString(")")
+}
+
 func DiffTree(expected, actual *Tree) []*TreeDiff {
 	if expected == nil && actual == nil {
 		return nil
diff --git a/spec/test/parser_test.go b/spec/test/parser_test.go
index 6e77f6d..41b7189 100644
--- a/spec/test/parser_test.go
+++ b/spec/test/parser_test.go
@@ -7,6 +7,25 @@ import (
 	"testing"
 )
 
+func TestTree_Format(t *testing.T) {
+	expected := `(a
+    (b
+        (c))
+    (d)
+    (e))`
+	tree := NewTree("a",
+		NewTree("b",
+			NewTree("c"),
+		),
+		NewTree("d"),
+		NewTree("e"),
+	)
+	actual := string(tree.Format())
+	if actual != expected {
+		t.Fatalf("unexpected format:\n%v", actual)
+	}
+}
+
 func TestDiffTree(t *testing.T) {
 	tests := []struct {
 		t1        *Tree
diff --git a/tester/tester.go b/tester/tester.go
index ef3ca61..70d4800 100644
--- a/tester/tester.go
+++ b/tester/tester.go
@@ -152,7 +152,7 @@ func runTest(g *gspec.CompiledGrammar, c *TestCaseWithMetadata) *TestResult {
 	}
 
 	// When a parse tree exists, the test continues regardless of whether or not syntax errors occurred.
-	diffs := tspec.DiffTree(genTree(tb.Tree()).Fill(), c.TestCase.Output)
+	diffs := tspec.DiffTree(ConvertSyntaxTreeToTestableTree(tb.Tree()).Fill(), c.TestCase.Output)
 	if len(diffs) > 0 {
 		return &TestResult{
 			TestCasePath: c.FilePath,
@@ -165,12 +165,12 @@ func runTest(g *gspec.CompiledGrammar, c *TestCaseWithMetadata) *TestResult {
 	}
 }
 
-func genTree(dTree *driver.Node) *tspec.Tree {
+func ConvertSyntaxTreeToTestableTree(dTree *driver.Node) *tspec.Tree {
 	var children []*tspec.Tree
 	if len(dTree.Children) > 0 {
 		children = make([]*tspec.Tree, len(dTree.Children))
 		for i, c := range dTree.Children {
-			children[i] = genTree(c)
+			children[i] = ConvertSyntaxTreeToTestableTree(c)
 		}
 	}
 	return tspec.NewTree(dTree.KindName, children...)