An intake air quantity control method for an internal combustion engine at idle. A supplementary air quantity to be supplied to the engine through a first auxiliary air passage bypassing an engine throttle valve is controlled by a first control valve on the basis of a basic value of a control amount corresponding to the difference between actual engine speed and desired idling speed. A supplementary air quantity to be supplied to the engine through a second auxiliary air passage is controlled by a second control valve in response to engine temperature. A first correction value is calculated based upon detected atmospheric pressure encompassing the engine, and a second correction value based upon detected engine temperature and atmospheric pressure, respectively. The basic value of the control amount is corrected by means of the first and second correction values, and the first control valve is driven according to the corrected control amount basic value. Preferably, the second correction value is calculated from a value of the control amount corresponding to a supplementary air quantity estimated to be supplied through the second control valve at the detected engine temperature and from the first correction value based upon the detected atmospheric pressure.