PURPOSE: To make the start at the unladen time and light loaded time positive by pressing a front side rear driving axle downward by an air bag means so as to enlarge the set bearing load of the front side rear driving axle, thereby enabling the axle load increase of the driving axle constantly within a range of avoiding the overload of each axle.

CONSTITUTION: Locker beams 12 for supporting a rear side rear driven axle 44 between the rear ends are oscillatingly supported to a trunnion 11 supported across side rails 42. Leaf springs 13 with the front ends supported to the side rails 42 of a frame 41 and with the rear ends connected to the front ends of the locker beams 12 through shackles 14 so as to support a front side rear driving axle 43 to the frame 41 are disposed in front of the trunnion 11. Shock absorbers 15 are disposed between the side rails 42 and the front side rear driving axle 43, and shock absorbers 16 are disposed between the side rails 42 and the rear side rear driven axle 44. An air bag means 17 is provided being supported to the frame 41 so as to constantly apply specified weight to the front side rear driving axle 42 in the depressing direction.