PURPOSE: To prevent occurrence of engine stall by reducing gradually the aux. air amount from the amount corresponding to the fully opened state of a control valve at the starting time of feedback control for the idling speed, and thereby smoothing the change of the number of engine revolutions.

CONSTITUTION: When it is judged (Step 3) that the engine is in cranking state, the open valve duty ratio DOUT of a control valve is set to 100% to supply much mixture gas to the engine. When a specific time tIU has passed since commencement of cranking, the process is proceeded to Step 10 so as to start the feedback control of the iding number of revolutions. In this feedback control, the aux. air amount is gradually substracted from the amount corresponding to the fully opened state of control valve. Thereby the change of the revolving speed of engine can be smoothed to ensure prevention of generation of engine stall.