To reduce the load applied to a front wheel and prevent partial wear of any tire on rear-front axle or rear-rear axle by mounting rear-front wheels or rear-rear wheels to the both ends of rear-front axle or rear-rear axle steerably, and suspending rear-front axle and rear-rear axle by front end side and rear end side of a leaf spring or beam.

Rear portion of a vehicle body frame 10 is supported by a rear-front axle 18 to have rear-front wheels 17 fitted to its left and right, and a rear-rear axle 20 to have rear-rear wheels 19 fitted to its left and right. A front air spring 55 and a rear air spring 56 are installed between a frame 10 and front end or rear end of a leaf spring 40 or a beam. By controlling the air pressure of the air springs 55 and 56 according to carrying load, the leaf spring 40 is constantly kept suspended in generally leveled state. This enables equal load distribution to the rear-front axle 18 and the rear-rear axle 20 which are apart longitudinally, and prevents slips between the rear-rear wheels 19 and road surface to ensure driving torque required for running.