The rise in popularity of communication toys specializing in voice feedback is noteworthy in recent years. However, it is also a fact that people quickly get tired of communication toys in comparison to conventional stuffed animals. On the other hand, stuffed animals which imitate conversation have a simple function of merely playing and parroting prerecorded user conversation. Since these are more popular than conversational stuffed animals, we cannot make a blanket statement that communication pattern is the cause thereof. Therefore, in this paper, of the entrainment phenomenon confirmed in human communication in which non-verbal information becomes aligned or synchronized, the fundamental frequency of the spoken voice was focused on. In order to artificially reproduce the entrainment phenomenon of the fundamental frequency, the fundamental frequency from the spoken voice of the user was calculated, and a fundamental frequency adjustment system which adjusts the fundamental frequency of the responding voice was constructed.