PURPOSE: To improve break-loose capability in an off-road by connecting a first and a second rotary shafts, which respectively transmit driving force to the front and rear wheels, through a vane pump which delivers oil in an amount in accordance with the difference of rotating speed of these rotary shafts.

CONSTITUTION: When a difference in roating speed is generated between an input shaft 24 and a front wheel output shaft 25 of a drive coupling device 23 for a four wheel drive to which driving force of a rear wheel output shaft connected to an engine through a speed change gear is transmitted, a rotor 26 makes a relative rotation with respect to a cam ring 27 and operating fluid in respective oil chambers 28 is pressurized by a vane 30. The pressurized operating oil flows from a delivery side chamber to a suction side chamber through an orifice of the vane 30 and is subject to resistance when it passes through the orifice. Because this passage resistance works in the direction for preventing the relative rotaion of the rotor 26 with respect to the cam ring 27, the difference of rotating speed between the rotor 26 and the cam ring 27 is reduced. In other words, driving force from the engine is distributed to the front wheel side and the rear wheel side, thereby a four wheel drive condition constantly rotating at an approximately equal speed is obtainable.