PURPOSE: To improve a drive force at the start of a vehicle by providing a pneumatic spring for regulating the ground contact pressure of a drive shaft to detect a change in the axial load for controlling the operation of the pneumatic spring.

CONSTITUTION: A drive shaft of two rear shafts is equipped with a start aiding apparatus 500 using a pneumatic spring 505 to increase the ground contact pressure of the drive shaft and the traction in the start or the like of a vehicle. A controller 800 supplies air to the pneumatic spring 505 when the empty vehicle is stopped, and exhausts the air when the loaded vehicle is stopped or moved with the speed exceeding the set vehicle speed. Two exhaust valves 2 are provided according to the requirement for exhausting air to control the opening and closing of two exhaust valves.