PURPOSE: To enhance the reliability and safety of an internal-combustion engine equipped with a supercharger, by determining the drive range and stop range of the supercharger on the basis of the parameters represented by an intake air quantity and a rotational speed of the engine, so as to check any excessive quantity of intake air supply.

CONSTITUTION: A detection signal Ne from a rotary angle sensor 3a and a detection signal Q from an air flowmeter 13 for implying an intake air quantity are input in a control computer 17 and when those signals belong to ON range Y, a start signal MgC is transmitted to a solenoid clutch 10 so as to operate an acceleration pump 6. The patterns comprise, besides, the first OFF range X in which the acceleration pump 6 is put in OFF position under the idling state or the state super low speed with low/intermediate loading, and the second OFF range Z in which the acceleration pump 6 is put in OFF position when a trouble in the acceleration pump 6 causes excessive rotational speed of the pump and/or when the engine shows its overspeed.