To increase starting performance by increasing ground-contact loads of drive wheels.

A hydraulic cylinder 11 is installed between front and rear axles and a frame on the upper side of the axles of a front biaxial vehicle. Hydraulic oil flowing from an oil pump 13 to a steering gear box 14 is switched by energizing a solenoid-operated first switching valve 16 to be extracted and led to the hydraulic cylinder 11 through a hydraulic oil supply line 17, and contracted to lift up the front and rear axles. To lower the front and rear axles, hydraulic pressure is merely released by energizing to open a solenoid-operated second switching valve 22 in a communication line 20. In normal traveling, the second switching valve 22 is closed again by deenergization, a pilot-operated shutoff valve 21 is returned to an open state by the dissipation of hydraulic pressure on the hydraulic oil supply line 17 side, and the hydraulic cylinder 11 is brought into a hydraulic pressure free state to absorb a suspension stroke.