diff --git a/Makefile b/Makefile
index 5d37121..8985019 100644
--- a/Makefile
+++ b/Makefile
@@ -5,3 +5,6 @@ test:
 
 fmt:
 	@go fmt ./...
+
+generate:
+	@go generate ./...
diff --git a/data.go b/data.go
index 59031d0..04da3c5 100644
--- a/data.go
+++ b/data.go
@@ -1,3 +1,5 @@
+// Code generated by tools/generator/generate.go; DO NOT EDIT.
+
 package regions
 
 var REGION_DATA = map[string]Region{
diff --git a/go.mod b/go.mod
index b27f256..f16be71 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module github.com/ohkinozomu/gcp-regions
 
 go 1.17
+
+require github.com/sanity-io/litter v1.5.2 // indirect
diff --git a/go.sum b/go.sum
index e69de29..e6ff270 100644
--- a/go.sum
+++ b/go.sum
@@ -0,0 +1,5 @@
+github.com/davecgh/go-spew v0.0.0-20161028175848-04cdfd42973b/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v0.0.0-20151028094244-d8ed2627bdf0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/sanity-io/litter v1.5.2 h1:AnC8s9BMORWH5a4atZ4D6FPVvKGzHcnc5/IVTa87myw=
+github.com/sanity-io/litter v1.5.2/go.mod h1:5Z71SvaYy5kcGtyglXOC9rrUi3c1E8CamFWjQsazTh0=
+github.com/stretchr/testify v0.0.0-20161117074351-18a02ba4a312/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
diff --git a/regions.go b/regions.go
index 39b8edf..1f37a58 100644
--- a/regions.go
+++ b/regions.go
@@ -1,3 +1,6 @@
+//go:generate go run tools/generator/generate.go
+//go:generate gofmt -w data.go
+
 package regions
 
 func IsValid(name string) bool {
diff --git a/tools/generator/generate.go b/tools/generator/generate.go
index ee864aa..dd082e5 100644
--- a/tools/generator/generate.go
+++ b/tools/generator/generate.go
@@ -3,10 +3,12 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
-	"fmt"
 	"io/ioutil"
+	"log"
 	"net/http"
+	"os"
 	"strings"
 
 	"github.com/sanity-io/litter"
@@ -19,23 +21,44 @@ type Region struct {
 	Longitude float64 `json:"longitude"`
 }
 
-func main() {
+func generate() (string, error) {
 	regionsJsonURL := "https://raw.githubusercontent.com/GoogleCloudPlatform/region-picker/main/data/regions.json"
 	resp, err := http.Get(regionsJsonURL)
 	if err != nil {
-		panic(err)
+		return "", err
 	}
 	defer resp.Body.Close()
 
 	regionData, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
-		panic(err)
+		return "", err
 	}
 
 	var m map[string]Region
 	if err := json.Unmarshal(regionData, &m); err != nil {
-		panic(err)
+		return "", err
 	}
 
-	fmt.Println(strings.Replace(litter.Sdump(m), "main.Region", "Region", -1))
+	return strings.Replace(litter.Sdump(m), "main.Region", "Region", -1), err
+}
+
+func main() {
+	var out bytes.Buffer
+	out.Reset()
+	out.WriteString(`// Code generated by tools/generator/generate.go; DO NOT EDIT.
+
+package regions
+
+var REGION_DATA = `)
+
+	generated, err := generate()
+	if err != nil {
+		log.Fatal(err)
+	}
+	out.WriteString(generated)
+
+	const target = "data.go"
+	if err := os.WriteFile(target, out.Bytes(), 0644); err != nil {
+		log.Fatal(err)
+	}
 }
diff --git a/tools/generator/go.mod b/tools/generator/go.mod
deleted file mode 100644
index 5697281..0000000
--- a/tools/generator/go.mod
+++ /dev/null
@@ -1,5 +0,0 @@
-module github.com/ohkinozomu/gcp-regions/tools/generator
-
-go 1.17
-
-require github.com/sanity-io/litter v1.5.2 // indirect
diff --git a/tools/generator/go.sum b/tools/generator/go.sum
deleted file mode 100644
index e6ff270..0000000
--- a/tools/generator/go.sum
+++ /dev/null
@@ -1,5 +0,0 @@
-github.com/davecgh/go-spew v0.0.0-20161028175848-04cdfd42973b/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/pmezard/go-difflib v0.0.0-20151028094244-d8ed2627bdf0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/sanity-io/litter v1.5.2 h1:AnC8s9BMORWH5a4atZ4D6FPVvKGzHcnc5/IVTa87myw=
-github.com/sanity-io/litter v1.5.2/go.mod h1:5Z71SvaYy5kcGtyglXOC9rrUi3c1E8CamFWjQsazTh0=
-github.com/stretchr/testify v0.0.0-20161117074351-18a02ba4a312/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=