PURPOSE: To enable to control a subsidiary throttle valve according to the quantity of intake air over the entire operational range of an engine and to obtain a good response at the time of transient operation of the engine, by controlling the opening of the subsidiary throttle valve according to the operational conditions of the engine.

CONSTITUTION: At the time of steady operation of an engine, the opening of a subsidiary throttle valve 10 is controlled by an actuator 13 on the basis of the first characteristics corresponding to the venturi vacuum related to the flow rate of intake air over the entire operational range of the engine. At the time of acceleration, loading of a spring of the actuator 13 is decreased through operation of a control means 15 which detects acceleration of the engine from the opening movement of a throttle valve 6. Thus, on the basis of the second characteristics, the subsidiary throttle valve 10 is controlled to open to a greater opening with respect to the venturi vacuum corresponding to the same flow rate of intake air.