When detecting a carrying quantity by a stroke position of the trunnion suspension at the back part of the vehicle body, to eliminate a hysteresis error by reciprocating an actuator by one stroke which moves up and down a driven wheel and temporarily increases the load distribution of a driving shaft when the vehicle starts.

A trunnion suspension has a hysteresis and produces different positions between after loading and after unloading even in the same carrying quantity. The hysteresis of a leaf sprint 9 is fixed to the extension side by one stroke of extension and shrinkage of an air cylinder 18. Therefore, since the detecting signal of a load cell 43 is read just after the air cylinder 18 extends and shrinks, the carrying quantity compared with a prescribed value is always detected under the same condition. Accordingly, since the detection error of the hysteresis need not be taken into account and thus improves the starting performance of a vehicle, it is possible to set the operating range of assisting the starting operation of the vehicle larger, such as increasing the quantity of load moving to the driving shaft.