PURPOSE: To make a transfer torque-relative speed characteristic controllable at will according to various conditions including a driving state and temperature or the like, by installing a draining opening and a valve capable of adjusting the opening of this opening both in an oil passage where discharge pressure out of an oil pump is led.

CONSTITUTION: The hydraulic fluid discharged out of an oil pump 50 is led into an oil chamber 35 from a discharge port 54 as in an arrow by way of an oil passage 36. Next, it is led into an oil passage 59 via a hole 57 of a cylinder 22 and a hole 58 of an output shaft 13, and the discharge pressure led into the opening 64 installed in a case 63 is controlled by a solenoid valve 65. In succession, the hydraulic fluid discharged out of this solenoid valve 65 is led into a clutch 30 from a drain hole 71 and, after lubricating this clutch 30, it is drawn into a suction port 53 of the oil pump 50. Here, if an input port 68 of the solenoid valve 65 is fully opened, discharge efficiency of the oil pump 50 comes to maximum, startup of the discharge pressure to the relative speed of both input and output shafts 11 and 13 comes to suddenness. In addition, opening of the solenoid valve 65 is made so as to be altered by a variation in a steering angle and oil temperature.