diff --git a/gluturtle.cabal b/gluturtle.cabal
--- a/gluturtle.cabal
+++ b/gluturtle.cabal
@@ -2,7 +2,7 @@
 cabal-version:	>= 1.6
 
 name:		gluturtle
-version:	0.0.14
+version:	0.0.15
 stability:	alpha
 author:		Yoshikuni Jujo <PAF01143@nifty.ne.jp>
 maintainer:	Yoshikuni Jujo <PAF01143@nifty.ne.jp>
diff --git a/src/Graphics/UI/GLUT/Turtle/Field.hs b/src/Graphics/UI/GLUT/Turtle/Field.hs
--- a/src/Graphics/UI/GLUT/Turtle/Field.hs
+++ b/src/Graphics/UI/GLUT/Turtle/Field.hs
@@ -362,9 +362,11 @@
 	writeIORef (fString f) [p]
 	continue <- ($ drop (length p) $ concat str) =<< readIORef (fInputtext f)
 	unless continue G.leaveMainLoop
-keyboardProc f (G.Char '\b') G.Down _ _ =
+keyboardProc f (G.Char '\b') G.Down _ _ = do
+	p <- readIORef $ fPrompt f
 	atomicModifyIORef_ (fString f) $ \s -> case s of
 		[""] -> [""]
+		[ss] | length ss <= length p -> s
 		s -> case last s of
 			"" -> init (init s) ++ [init $ last $ init s]
 			_ -> init s ++ [init $ last s]