1. A robot apparatus whose moving parts are controlled to make the robotic device behave expressively, the device comprising: means for detecting external or internal information; means for acquiring a cause factor influencing the behavior from the external or internal information detected by the information detecting-means; means for acquiring an occurrence tendency of the cause factor-influenced behavior based on the cause factor acquired by the cause factor acquiring means; means for making a comparison among occurrence tendencies of two or more behaviors, acquired by the occurrence tendency acquiring means and belonging to the same group; means for selecting one of the behaviors based on the result of the occurrence tendency comparison made by the occurrence tendency comparing means; and means for controlling the moving parts based on the behavior selected by the behavior selecting means to have the robot apparatus express the selected behavior; the occurrence tendency of the behavior selected by the behavior selecting means being varied adaptively to the cause factor which is variable due to the actual occurrence of the behavior.