An air-fuel ratio control apparatus for an internal combustion engine comprises a carburetor for controlling the air-fuel ratio of the fuel-air mixture supplied to the engine, a thermal reactor in which the exhaust gas from engine is caused to undergo reaction with secondary air supplied to the reactor, an oxygen sensor disposed in an exhaust gas passage between the reactor and three-way catalyst, a negative pressure generator for producing a negative pressure corresponding to the output from the oxygen sensor, a detector for detecting a low and a high speed operation region of the engine, and means for controlling a low speed fuel system and a main fuel system, respectively, of the carburetor in response to the negative pressure in such a manner that the fuel-air mixture is enriched with the secondary air supply being concurrently increased in the low speed operation region of the engine, while in the high speed operation range the fuel-air mixture having an approximately stoichiometric air-fuel ratio is supplied to the engine. A method of controlling the air-fuel ratio controlling apparatus is also disclosed.