diff --git a/internal/uidriver/mobile/ui.go b/internal/uidriver/mobile/ui.go
index 69f05e4a7..4856688b8 100644
--- a/internal/uidriver/mobile/ui.go
+++ b/internal/uidriver/mobile/ui.go
@@ -68,6 +68,8 @@ func Get() *UserInterface {
 }
 
 // Update is called from mobile/ebitenmobileview.
+//
+// Update must be called on the rendering thread.
 func (u *UserInterface) Update() error {
 	select {
 	case err := <-u.errCh:
@@ -358,6 +360,8 @@ func (u *UserInterface) ScreenSizeInFullscreen() (int, int) {
 }
 
 // SetOutsideSize is called from mobile/ebitenmobileview.
+//
+// SetOutsideSize is concurrent safe.
 func (u *UserInterface) SetOutsideSize(outsideWidth, outsideHeight float64) {
 	// Called from ebitenmobileview.
 	u.m.Lock()
diff --git a/mobile/ebitenmobileview/mobile.go b/mobile/ebitenmobileview/mobile.go
index a9bd03095..b1b6a95dc 100644
--- a/mobile/ebitenmobileview/mobile.go
+++ b/mobile/ebitenmobileview/mobile.go
@@ -27,7 +27,7 @@ import "C"
 
 import (
 	"runtime"
-	"sync"
+	"sync/atomic"
 
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/internal/restorable"
@@ -37,33 +37,26 @@ import (
 var theState state
 
 type state struct {
-	started bool
-
-	// m is a mutex required for each function.
-	// For example, on Android, Update can be called from a different thread:
-	// https://developer.android.com/reference/android/opengl/GLSurfaceView.Renderer
-	m sync.Mutex
+	running int32
 }
 
 func (s *state) isRunning() bool {
-	return s.started
+	return atomic.LoadInt32(&s.running) != 0
+}
+
+func (s *state) run() {
+	atomic.StoreInt32(&s.running, 1)
 }
 
 func SetGame(game ebiten.Game) {
-	theState.m.Lock()
-	defer theState.m.Unlock()
-
-	if theState.started {
+	if theState.isRunning() {
 		panic("ebitenmobileview: SetGame cannot be called twice or more")
 	}
 	ebiten.RunGameWithoutMainLoop(game)
-	theState.started = true
+	theState.run()
 }
 
 func Layout(viewWidth, viewHeight float64) {
-	theState.m.Lock()
-	defer theState.m.Unlock()
-
 	mobile.Get().SetOutsideSize(viewWidth, viewHeight)
 }
 
@@ -72,9 +65,7 @@ func Update() error {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	theState.m.Lock()
-	defer theState.m.Unlock()
-	if !theState.started {
+	if !theState.isRunning() {
 		// start is not called yet, but as update can be called from another thread, it is OK. Just ignore
 		// this.
 		return nil