diff --git a/mobile/impl_mobile.go b/mobile/impl_mobile.go
index b5b3f6c2e..0a2539628 100644
--- a/mobile/impl_mobile.go
+++ b/mobile/impl_mobile.go
@@ -18,22 +18,17 @@ package mobile
 
 import (
 	"errors"
-	"sync"
 
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/internal/ui"
 )
 
 var (
-	chError     <-chan error
-	running     bool
-	mobileMutex sync.Mutex
+	chError <-chan error
+	running bool
 )
 
 func update() error {
-	mobileMutex.Lock()
-	defer mobileMutex.Unlock()
-
 	if chError == nil {
 		return errors.New("mobile: chError must not be nil: Start is not called yet?")
 	}
@@ -44,9 +39,6 @@ func update() error {
 }
 
 func start(f func(*ebiten.Image) error, width, height int, scale float64, title string) {
-	mobileMutex.Lock()
-	defer mobileMutex.Unlock()
-
 	running = true
 	chError = ebiten.RunWithoutMainLoop(f, width, height, scale, title)
 }
diff --git a/mobile/mobile.go b/mobile/mobile.go
index f8ce280a1..87774af85 100644
--- a/mobile/mobile.go
+++ b/mobile/mobile.go
@@ -21,17 +21,31 @@
 package mobile
 
 import (
+	"sync"
+
 	"github.com/hajimehoshi/ebiten"
 )
 
+var (
+	// mobileMutex 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
+	mobileMutex sync.Mutex
+)
+
 // Start starts the game and returns immediately.
 //
 // Different from ebiten.Run, this invokes only the game loop and not the main (UI) loop.
 //
 // The unit of width/height is device-independent pixel (dp on Android and point on iOS).
 //
+// Start is concurrent-safe.
+//
 // Start always returns nil as of 1.5.0-alpha.
 func Start(f func(*ebiten.Image) error, width, height int, scale float64, title string) error {
+	mobileMutex.Lock()
+	defer mobileMutex.Unlock()
+
 	start(f, width, height, scale, title)
 	return nil
 }
@@ -43,8 +57,13 @@ func Start(f func(*ebiten.Image) error, width, height int, scale float64, title
 //
 // On iOS, this should be called at glkView:drawInRect: of GLKViewDelegate.
 //
+// Update is concurrent-safe.
+//
 // Update returns error when 1) OpenGL error happens, or 2) f in Start returns error samely as ebiten.Run.
 func Update() error {
+	mobileMutex.Lock()
+	defer mobileMutex.Unlock()
+
 	return update()
 }
 
@@ -76,8 +95,13 @@ func Update() error {
 //
 // The coodinate x/y is in dp.
 //
+// UpdateTouchesOnAndroid is concurrent-safe.
+//
 // For more details, see https://github.com/hajimehoshi/ebiten/wiki/Android.
 func UpdateTouchesOnAndroid(action int, id int, x, y int) {
+	mobileMutex.Lock()
+	defer mobileMutex.Unlock()
+
 	updateTouchesOnAndroid(action, id, x, y)
 }
 
@@ -113,7 +137,12 @@ func UpdateTouchesOnAndroid(action int, id int, x, y int) {
 //
 // The coodinate x/y is in point.
 //
+// UpdateTouchesOnIOS is concurrent-safe.
+//
 // For more details, see https://github.com/hajimehoshi/ebiten/wiki/iOS.
 func UpdateTouchesOnIOS(phase int, ptr int64, x, y int) {
+	mobileMutex.Lock()
+	defer mobileMutex.Unlock()
+
 	updateTouchesOnIOSImpl(phase, ptr, x, y)
 }