The purpose of this paper is to construct a methodology for smooth communications between humans and robots. Here, focus is on a mindreading mechanism, which is indispensable in human-human communications. We propose a model of utterance understanding based on this mechanism. Concretely speaking, we apply the model of a mindreading system [Baron-Cohen 96] to a model of human-robot communications. Moreover, we implement a robot interface system that applies our proposed model. Psychological experiments were carried out to explore the validity of the following hypothesis: by reading a robot’s mind, a human can estimate the robot’s intention with ease, and, moreover, the person can even understand the robot’s unclear utterances made by synthesized speech sounds. The results of the experiments statistically supported our hypothesis.