PURPOSE: To stably control the transmission torque of a rotational difference sensitive coupling in a region where the control by an orifice opening is unstable by another technique.

CONSTITUTION: While the power from final drive gear sets 3, 4 is output to side flanges 9, 10, if the rotational difference is produced between the side flanges, a piston 12 is stroked by a cam face 14, working oil of an accumulator chamber 16 is forced to flow into a piston chamber 13 through a check valve 15, and the working fluid is reversely returned from the chamber to the chamber 16 through a variable orifice 17. The piston 12 and the cam face 14 are enabled to transmit the torque between the side flanges 9, 10 by limiting the flow of working fluid by the orifice 17. The transmission torque can be controlled by adjusting the internal pressure of the accumulator chamber 16 by a pressure regulator.