PURPOSE: To always control low speed travelling at a proper amount of control irrespective of the operating condition of a supercharger by providing a control constant decision means to differentiate the control constant according to parameters for the operation and non-operation of the supercharger.

CONSTITUTION: A control device 6 measures a pulse frequency by means of pulse train signals from a vehicle speed sensor 3 to obtain a vehicle speed Vs, receives throttle opening voltage signals from a throttle opening sensor 28 according to the opening of a throttle to make, for example, A/D conversion, and makes mapping using a calculated vehicle speed V and a detected throttle opening to judge whether the vehicle speed is within turbocharger area or not. If it is within the turbocharger area, determines the control device 6 a control constant at, for example, O.5 and, if it is outside the turbocharger area, determines it at, for example, 1.0. Then a target speed Vs is compared with an actual vehicle speed Vr and, using the determined control constant, a control pulse width for driving a throttle valve 9 so that a vehicle can travel at the target speed Vr is calculated.