To set widely a range in which a drive force can be applied and, at the same time, to suppress a reduction in a drive performance to a little degree by independently controlling in response to the internal pressure of an air spring and vehicle weight the internal pressure of the air spring for suspending a rear front axle and the internal pressure of the air spring for suspending a rear rear axle in driving force applied condition.

A rear front axle 17 is hanged with a leaf spring 21, and an air spring 23. Furthermore, the rear rear axle 18 is hanged with a leaf spring 22, and an air spring 24 respectively. The rear front axle 17 constitutes a drive shaft while the rear rear axle 18 constitutes a follower shaft. The front and the rear air springs 23 and 24 are communicated with a communication pipe 27, and the vehicle height is detected with a bearing sensor 41 attached on the rear rear axle 18, and at the same time, the computer 50 controls a levelling valve 40 in accordance with the detection output to control a supply and an exhaust of air of a front and rear air springs 23 and 24. As a consequence, the range of the vehicle weight for applying a drive force can be set widely, and, at the same time, a lowering of an inclination of an asendable slope can be suppressed.