— This paper describes designing expression of robot’s primitive minds in a simple and effective way. Although expression of robot’s minds like internal states is becoming a major topic in human-robot interaction, few studies to propose a policy to design concrete expression. In this paper, we propose the design policy, called “SE2PM: Simple Expression to Primitive Mind”, and fully implement expression of robot’s minds based on the design policy, then investigate the effectiveness of them. SE2PM implies that intuitive and simple expression like beep sound from a simple robot (e. g. a mobile robot) is more effective than complicated behaviors from a complex robot (e. g. a doglike pet robot) in informing its primitive minds to a human. Thus, in order to investigate the validation of our SE2PM policy, we implement two robots: a mobile robot based on Mindstorms with mind expression of beep sound, and a pet robot, AIBO, with mind expression of complicated behaviors. We also cond...