This paper introduces a passive dance partner robot for realizing the effective human-robot cooperation. Different from the active-type robot with servo motors, the passive robot is controlled by the servo brakes attached to the wheels of the robot and its driving force is the actual force/moment applied by a human. We also propose a control architecture referred to as CAST (Control Architecture for Step Transition) for passive dance partner robot. This paper especially focuses on the Motion Generator in CAST. The Motion Generator controls the brake torque of each wheel with servo brake attached to the mobile base of the robot and realizes the ballroom dancing of the robot together with a human partner.