An intake air amount control system for an internal combustion engine installed in an automotive vehicle has an intake air amount control valve arranged in a bypass passage bypassing a throttle valve arranged in an intake passage of the engine. The intake air amount control valve controls the amount of air supplied to the intake passage at a location downstream of the throttle valve. The intake air amount control valve is controlled in dependence on operatives states of electric devices including an electrically-heated catalyzer which are installed on the vehicle and supplied with electric power by an alternator installed on the vehicle and driven by the engine, when the engine is in a predetermined operating condition. The intake air amount control valve is controlled by the use of a correction amount dependent on an operative state of the electrically-heated catalyzer.