PURPOSE: To obtain large output torque even if the difference of the rotation between an input shaft and an output shaft by providing a speed increasing mechanism which increases speed and charges the difference of the rotation between an input shaft and an output shaft to transmit it to a pump impeller.

CONSTITUTION: An input shaft 43 is rotated by rotary driving force from a transfer 18 to rotate a pump impeller 49 via a speed increasing mechanism 56, operating oil in a circuit 51 is circulated in the direction of an arrow by the rotation of the impeller 49, an turbine impeller 37 is thereby rotated, torque is transmitted to an output shaft 35 and driving force is transmitted to a rear wheel 33 side via a propeller shaft 27. The difference of rotation between a front and a rear wheel 23, 33 is generated to be transmitted to the pump impeller 49 as its speed is increased by a speed increasing mechanism 65. Since the direction of the rotary force is in the direction for increasing the difference of the rotation of the impeller 49 with respect to the impeller 37, large torque as well as the difference of the rotation is amplified is transmitted to the impeller 37 for driving an output shaft 35.