PURPOSE: To improve a smooth starting characteristic and rotational stability after starting, by setting an AAC valve interposingly installed in a bypass passage, bypassing a carburetor throttle valve, to the specified value (full-open) in time of starting, while making it close by slow degrees simultaneously with the elapse of time in time of an end of starting.

CONSTITUTION: An AAC valve 4 is interposingly installed in a bypass passage 5 being installed so as to bypass a carburetor throttle valve 2, controlling its opening, whereby an idling speed is controlled. And this valve 4 is controlled together with an EGR control valve 6 via each of solenoid valves 7 and 8 operated by a signal out of a controller 18. On the other hand, this controller 18 inputs various operating state detection signals S1WS8 and in time of starting, it is so constituted as to control the ACC valve 4 to make it full open. In addition, in time of an end of starting, a bypass air flow rate is set down to a first fixed value corresponding to cooling water temperature and then this controller is constituted so as to make opening of the AAC valve 4 controllable as causing a second fixed value dependent upon the cooling water temperature to be decreased up to the specified value by degrees.