An air-fuel ratio control system for an internal combustion engine having an intake passage, an exhaust passage, an air-fuel mixture supply device, an on-off type electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply device, a dither signal generating circuit for producing a periodical dither signal, a shift control circuit for shifting the level of the center of the dither signal, a driving circuit for producing a driving output for the on-off type electromagnetic valve, and an O.sub.2 sensor for detecting the concentration of oxygen in exhaust gases passing through the exhaust passage. A timing circuit is provided for detecting the period of the time when the output of the O.sub.2 sensor is higher than a predetermined level and the period of the time when the output is lower than the predetermined level for producing an output signal in dependency on the difference between the periods. A shift signal generating circuit is provided for generating a shift signal in dependency on the output of the timing circuit. The shift control circuit is so arranged to control the air-fuel ratio of the mixture in such a direction that the difference is decreased.