PURPOSE: To enable interior circulating torque to be absorbed in turning with four wheel drive in a transfer clutch having variable transmission torque for changing over two wheel drive to four one or vice versa by maximizing the tramsmission torque in slip.

CONSTITUTION: Small and large diameter pistons 33, 35 are incorporated in a transfer clutch 22 at both sides of a pressure plate 30 to vary engaging force between clutch plates 27, 29 by oil pressure in piston chambers 34, 36. Also, to the input of a control circuit 46 for driving solenoid valves 39, 40 between the chambers 34, 36 and an oil pump 38 are supplied signals of rotation sensors 42, 43 of front and rear wheels, a manual change-over switch 4 for changing over wheel drive to two wheel drive and a load sensor 45. The pressing force of the plate 30 is varied with load by the signal of thesensor 45 and when slip ratio is judged high by the signals of sensors 42, 43 a clutch 22 is integrally engaged by the valve 40 to provide stable travelling.