Hirata and Whiton [2005] revealed that the production boundary between a single and a geminate stop in Japanese is invariant over speaking rates in terms of the ratio of stop closure duration to word duration (closure-word ratio). This study addressed the question of whether the ratio is also invariant for the perceptual boundary. An experiment was conducted to obtain the perceptual boundary between a single and a geminate stop at slow, normal and fast speaking rates. The results showed that the closure-word ratio at the perceptual boundary did not coincide with that at its production boundary. However, the closure-word ratio was consistent within each stimulus item for all speaking rates, although it was different among the stimulus items. The results suggest that the closure-word ratio at the perceptual boundary is invariant over speaking rates within an item, but some item-related factors affect it.