PURPOSE: To improve the steering responsiveness with possible stability for steering by automatically controlling in accordance with a car speed and a steering angle, based on a speed signal of right and left front wheels, an operating pressure by control air of differential limiting torque selected to the differential gear lock operation by a manual switch.

CONSTITUTION: When differential limiting action is performed in accordance with a steering angle at the time of normal running, a manual switch 56 is turned off controlling by a car speed and a steering angle the first and second solenoid control valves 52, 54 by a controller 55 controlling an operating pressure supplied to an air cylinder 27 in accordance with the steering angle. A signal is input to the controller 55 by right and left front wheel speed sensors 57a, 57b and used as the car speed in the mean value of right and left front wheel speeds. The steering angle is calculated in the controller 55 by the car speed and a differential speed between right and left front wheels. A control valve 52 controls a pressure of air from an air tank 50 to a predetermined value in accordance with the car speed, and a control valve 54 controls the operating pressure supplied to the cylinder 27 by adjusting, in accordance with the steering angle an amount of the air discharged, to the atmosphere controlled to the predetermined pressure.