PURPOSE: To control, always with high precision, the air-fuel ratio of an internal combustion engine of the type in which the opening periods of intake and exhaust valves and the lifting amount thereof are variable, by controlling the control gain of a feedback control in accordance with the volume of blow-by gas obtained from the difference between the charged volume of the mixture into engine cylinder and the volume of new intake air.

CONSTITUTION: The output of an oxygen sensor 34 is received by a feedback control circuit 40 which produces through proportional and integrating control, driving pulses for solenoid valves 37A, 37B controlling the mixture in order that it has a stoichiometric air-fuel ratio. The control circuit comprises a deviation detecting circuit 41, a wave shaping circuit 42 and a driving circuit 43 for amplifying the output of the wave shaping circuit 42 and producing signals for driving the solenoid valves 37A, 37B. The control gain of feedback control is changed by foreknowning fluctuations in the air- fuel ratio, and the reduction volume of blow-by gas is obtained from the detected difference between the intake air volume passing through an intake air system and the intake air volume charged into engine cylinders through intake valves so that at least one of proportional and integrating constants is changed in accordance with the reduction volume, thereby the control having satisfactory responce may be performed.