To eliminate deterioration of emission by instantaneously controlling the opening and closing of a flow control valve to a preset designated opening according to the change of detected load to an engine, and then prohibiting the instantaneous opening and closing control until designated time elapses even if the load condition is changed.

An electronic control device 16 incorporates a program for changing the step of a step motor 11 for driving a valve element 9 of a flow control valve 8, taking a voltage signal (a) output by an O2 sensor 15 and an analog voltage signal (b) output by a throttle sensor 21 as main information, and conducting air fuel ratio feedback control in such a manner as to reach the calculated air fuel ratio corresponding to the operating condition of an engine. In the above program, according to the change of the detected load to the engine, the flow control valve 8 is instantaneously skip-controlled to a preset designated opeing, and then the skip control is prohibited until designated time elapses even if the condition of load is changed.