An air-fuel ratio control system for an automotive internal combustion engine provided with a three-way catalytic converter disposed in an exhaust gas passageway of the engine. The air-fuel ratio control system is comprised of an oxygen sensor disposed in the exhaust gas passageway downstream of the catalytic converter. The air-fuel ratio of air-fuel mixture to be supplied to the engine is controlled toward a stoichiometric value to cause the three-way catalytic converter to efficiently work under the action of a control unit including a microcomputer. In the control unit, a control constant is set in accordance with the output voltage of the oxygen sensor, and a cycle signal (pulse signal) is cyclically generated. The control constant is compulsorily minutely vibrated under the action of the cycle signal, thereby obtaining an air-fuel ratio correction coefficient. The amount of fuel to be supplied to the engine is calculated in accordance with the air-fuel ratio correction coefficient and injected from a fuel injector valve.