To regulate oscillation of an axle to a body frame more positively.

On both lateral sides of a rear beam 2 oscillatingly supported to a body frame 1, the body frame 1 and rear axle beam 2 are respectively connected to hydraulic cylinders 6, 7. The head side pressure receiving area and rod side pressure receiving area of pistons of the respective hydraulic cylinders 6, 7 are made equal. Lines 10a, 10b, 14a, 14b communicating rod side oil chambers 23 and head side oil chambers 24 of the hydraulic cylinders 6, 7 are provided with solenoid control valves 8, 12 allowing or impeding movement of hydraulic oil between both oil chambers 23, 24. As oscillation control means 16 controls the respective solenoid control valves 8, 12 on the basis of a specified axle fixing condition so as to impede movement of hydraulic oil between both oil chambers 23, 24 of the hydraulic cylinders 6, 7.