In an air-fuel ratio control method and apparatus for an internal combustion engine supplied with a gas mixture of air and fuel, in which an actual air-fuel ratio is determined from exhaust gas components by using an O.sub.2 -sensor installed in an exhaust pipe of the engine, and the air-fuel ratio of the mixture gas to be supplied to the engine is controlled through feedback control on the basis of deviation of the actually detected air-fuel ratio from a reference value (A/F=14.7). Performance of the O.sub.2 -sensor is determined by making use of change in the air-fuel ratio output of the O.sub.2 -sensor at a time of interruption of the fuel supply to the engine or at a time the fuel supply to the engine is restarted.