A main purpose of our research is realizing an effective human-robot cooperation with physical interaction. In this paper, an architecture for the cooperation is proposed. As an example of the cooperation, ballroom dancing by a human and a robot is focused. And a dance partner robot referred to as “MS DanceR (Mobile Smart Dance Robot) ” is developed. MS DanceR realizes a ballroom dance together with a human based on a control architecture referred to as “CAST (Control Architecture based-on Step Transition) ”, which is designed according to features of ballroom dances. Experimental results illustate the validity of MS DanceR and its control architecture CAST.