Personal robots, which are expected to become popular in the near future, are required to be active in work and community life alongside humans. Therefore, we have been developing new mechanisms and functions in order to realize natural bilateral interaction by expressing emotions, behaviors and personality in a human-like manner. We have proposed a mental model with emotion, mood, personality and needs. However, the robot behavior was very simple since the robot shows just a single kind of behavior in response to a robot mental state. In this paper, we present a new behavior model for humanoid robots based on operant conditioning, which is a well-known psychological behavior model. We implemented this new behavior model into the Emotion Expression Humanoid Robot WE-4RII (Waseda Eye No.4 Refined II), developed in 2004. Through the experimental evaluations, we confirmed that the robot with the new behavior model could autonomously select suitable behavior for the situation within a predefined behavior list.