PURPOSE: To obtain a required torque transmission characteristic all the time by displacing a partition member for dividing an operating chamber at the time of forward rotation relatively to at the time of reverse rotation, and providing a means for changing over an oil passage led to the operating chamber by utilizing the relative displacement.

CONSTITUTION: Upon forward rotation of a propeller shaft 1, torque is applied to a hydraulic coupling 3 so that a rotational difference is generated between an input shaft 6 and an output member 10. Accordingly, oil is discharged to a high pressure side by pumping action caused by engagement of an inner gear 15 with a pinion 17. The torque is transmitted by the discharge pressure serving as drive resistance. In this case, a partition member 22 for dividing an operating chamber 20 into a high pressure portion and a low pressure portion is pushed by an oil pressure of the high pressure portion to be displaced, thus sealing the low pressure portion. Upon reverse rotation of the shaft 1, the pinion 17 is rotated reversely, thus reversing a suction side and a discharge side. The partition member 22 is pushed toward the side opposite to that at the time of the forward rotation, to be displaced, thereby sealing the lower pressure portion, and an oil passage to the operating chamber 20 is changed over. Therefore, the oil passage can be easily changed over, and a desired torque transmission characteristic can be obtained all the time.