This study reports a perceptual effect of a devoiced vowel in Japanese as an assimilative outcome of connected speech. A focus is placed on how such an effect changes its magnitude as the source and the recipient of the effect are separated. A devoiced syllable ki- has a strong effect of causing voiceless judgment in the following da-ta VOT variations with natural closure durations. However, such an effect is weakened as the closure interval is expanded beyond the original one for a voiceless stop. In this case, the original voicing judgment of the da-ta variations in isolation gradually returns. Also, the more voiced the da-ta variation itself, the earlier and farther it deviates from the influence of the devoiced vowel. Phonetic variation resulting from connected speech serves to aid segmental perception, but in turn the effect obtains in a connected speech event.