To make compact a mechanism of racing at the time of driving a two wheel by rotatably housing an inner differential case provided with a bevel gear type differential gear mechanism in an outer differential case and housing a clutch member which is integrally rotated with the outer differential case in a manner movable in an axial direction.

In a four wheel drive state in which a predetermined pressure is supplied to a pressure chamber 65a of a main body 65 of an actuator in a front differential 13, a diaphragm 65b is deviated against a return spring 63 so that the dog clutches 41a and 43a of the inner differential case 41 and the clutch member 43 engages with each other, and the outer differential case 31 and the inner differential case 41 inside of the outer differential case 31 are integrally rotated. However, when a pressure in the pressure chamber 65 is removed, the diaphragm 65b returns to the left, and the clutch member 43 is separated with the result that each of the differential cases 31 and 41 can rotated independently. Consequently, only the rear wheel can be rotated only with the rear wheel of the power of the engine.