PURPOSE: To restrict the discharge of unburned HC at the time of starting an engine, while enabling the engine speed to rise smoothly.

CONSTITUTION: An intake air flow control valve 9 to be driven by a negative pressure diaphragm type driving device 21 is arranged in an intake air branch 5. At the time of cranking an engine, the intake air flow control valve 9 is held in the closed condition, and when the engine speed exceeds the predetermined speed, for example, at 400r.p.m, the valve opening signal for opening the intake air flow control valve 9 is given to the negative pressure diaphragm type driving device 21. After giving the valve opening signal, during the valve opening delay period of the intake air flow control valve 9, the opening area of an idling speed control valve 13 is increased.