Abstract: Disclosed is an air-fuel control device for an internal combustion engine which includes: an O.sub.2 sensor for detecting the oxygen concentration of exhaust gas; a control circuit having a comparator for comparing the output voltage from the O.sub.2 sensor with a reference voltage (V.sub.R), so that the air-fuel ratio of the mixture discriminated as rich or lean with respect to a stoichiometric air-fuel ratio, and; an installation for supplying fuel into an intake passage, so that when the air-fuel ratio of the mixture is leaner than the stoichiometric value the amount of fuel is increased and when the air-fuel ratio of the mixture is richer than the stoichiometric value the amount of fuel is decreased. This device further includes a sensor for detecting the accelerating operating conditions of the engine, and a mechanism for changing the reference voltage (V.sub.R) to a value (V.sub.R2) different from a predetermined value (V.sub.