A fricative [s] and an affricate [ts] pronounced by both native Japanese and Korean speakers were analyzed to clarify the effect of the mother language on speech production. It was revealed that Japanese speakers have a clear individual production boundary between [s] and [ts], and that this boundary corresponds to the production boundary of all Japanese speakers. In contrast, although Korean speakers tend to have a clear individual production boundary, the boundary dose not corresponds to that of Japanese speakers. These facts suggest that Korean speakers tend to have a stable [s]-[ts] production boundary but that differ from Japanese speakers.