diff --git a/event/modifier.go b/event/modifier.go
index aeb9c37bb..b44c14d85 100644
--- a/event/modifier.go
+++ b/event/modifier.go
@@ -18,6 +18,7 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/driver"
 )
 
+// Modifier is a bit set of modifier keys on a keyboard.
 type Modifier int
 
 const (