An exhaust gas purifying system for internal combustion engines includes a three-way catalyst disposed in the exhaust system of the engine, and secondary air is supplied to the exhaust system through an air control valve under the control of an air-fuel ratio control circuit. The air-fuel ratio control circuit produces a control signal which increases or decreases stepwisely at every reception of a clock signal depending on whether the air-fuel ratio in the exhaust system is above or below a stoichiometric air-fuel ratio. The air control valve is of the proportional electromagnetic type and controls the secondary air passing therethrough in proportion to the control signal.