An air-fuel ratio control system for an internal combustion engine which comprises 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 first circuit is provided for detecting a middle value between the maximum value and minimum value of the output voltage of the O.sub.2 sensor in each cycle, and a second circuit is provided for detecting a middle value with respect to the output waveform of the O.sub.2 sensor such that the areas thereof above and below the latter middle value are equal. A shift signal generating circuit is provided for comparing the outputs of the first and second circuits for generating the difference as a shift signal. 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.