A behavior decision system (70) includes a perceptual information acquisition unit (90) which acquires a cause factor being external or internal information acquired by a CCD camera (20), distance sensor (22), microphone (23) or the like and which influences a behavior and a motivational information acquisition unit (81) which acquires an occurrence tendency of a behavior influenced by the cause factor based on the cause factor from the perceptual information acquisition unit (90), a behavior selecting processor (82) which compares occurrence tendencies corresponding to two or more behaviors, acquired by the perceptual information acquisition unit (90) and motivational information acquisition unit (81) and belonging to the same group, to thereby select one of the behaviors, and an output semantics converter module (68) which controls moving parts based on the behavior selected by the behavior selecting processor (82) for expressing the selected behavior. With the behavior decision system, there can be provided a robot having an improved likeness to a living thing or a creature and showing a more similar behavior to that of an animal.