To adjust relative driving speed between front wheels and rear wheels by providing a means automatically connecting a drive device from a hydraulic motor to the front wheels, when a hydraulic motor and the steering angle of the front wheels exceed a first prescribed value.

An electronic steering angle sensor 21 fitted to the king pin of a front wheel is connected to an electronic control device 22. When the steering angle exceeds 15 degrees, the electronic control device 22 sends a signal to a four-wheel drive clutch actuator 24, so as to detach the four-wheel drive clutch 8 from interlocking and open a flow to a hydraulic motor 11. Then, drive from a main transmission 2 is disconnected from the drive shaft 7 of a front wheel axle, rotating speed of the drive shaft 7 of the front wheel axle is decided by the ground speed or drive from the motor 11. Torque of the motor 11 is adjusted by a control valve 20, changed within the limited position of the valve avoiding slip of the wheels, and general activity of the drive device against the front wheels is controlled.