A four wheel drive system of a vehicle has a transfer clutch capable of varying clutch torque transmitted therethrough from an engine to front wheels. The clutch torque is controlled by a driving force distribution control unit in accordance with a front and rear wheel speed difference determined from signals from front and rear wheel speed sensors. Preferably, the control unit controls a distribution ratio of a front wheel driving force to a rear wheel driving force by controlling the clutch torque of the transfer clutch in such a manner that the distribution ratio decreases as the wheel speed difference increases when the wheel speed difference is positive, that is the front wheel speed is higher than the rear wheel speed, and that the distribution ratio increases as the absolute value of the wheel speed difference increases when the wheel speed difference is negative.