PURPOSE: To smoothly rotate an output shaft without rotational difference when the engine speed is increased by providing a disc having a circular space, an eccentric circular plate in the interior thereof and a plurality of hydraulic cylinders on the disc, thereby controlling flowing oil.

CONSTITUTION: A disc 2 is rotated in the same speed by rotation of an input shaft 1, and a piston 9 of a hydraulic cylinder 10 of the disc 2 is projected and sunk along an eccentric circular plate 7, whereby oil is let flow in a fixed direction through a communicating passage by the pumping action. During blow rotation, however, the disc 2 is only rotated on the outside of the eccentric circular plate 7 and power is not transmitted. When the rotating, speed is increased, a spool 16 of a control valve 14 is moved outside by centrifugal force, so that the piston 9 is hard to project end sink and the eccentric circular plate 7 is pressed to be rotated by rotation of the disc 2 through the piston 7. When it reaches further high speed rotation, the control valve 14 completely checks flowing of oil, so that the eccentric circular plate 7 is locked by the piston 9 and rotated at the equal speed to that of the disc 2 to transmit power to an output shaft.