PURPOSE: To change the rotational speed on a driven shaft side by controlling the discharge pressure and the flow rate of the working fluid to be sucked/ discharged in/from an enclosed space with an inner rotor which is rotated and revolved in an internally contact manner with an inner circumferential tooth of an outer rotor.

CONSTITUTION: An outer rotor 2 is rotated by rotation of a driving shaft 10 under rotation of a motor shaft PM, and the working oil sucked into a trochoidal tooth type fluid mechanism TC from an inlet port 6 enters enclosed spaces 20a/b between rotors. When a throttle valve Vd provided on the discharge side is throttled, the back pressure is applied to the working fluid of the inner rotor 1, the rotational torque is applied to the inner rotor 2, and gradually revolved while generating the rotational delay to the outer rotor 2. As a result, the rotational force is given to a driven shaft 5 to which a load is connected. When the throttle valve Vd is further throttled, the outer rotor 2 and inner rotor 1 of the driving shaft 10 are restricted with the working oil to allow the same number of revolution of both rotors.