Personal robots, which are expected to become popular in the future, are required to be active in joint work and community life with human. Therefore, the objective of this study is the development of new mechanisms and functions for a humanoid robot to express emotions and to communicate naturally with human. We developed both the mental model from psychological point of view and the Emotion Expression Humanoid Robot WE-4RII (Waseda Eye No. 4 Refined II) from engineered point of view. In this paper, a co-associative memory model using mutually coupled chaotic neural networks was proposed and implemented in WE-4RII as its mental model. We confirmed that the robot could generate the behavior depending on its mood in response to a stimulus.