AIR-FUEL RATIO CONTROL SYSTEM AND AIR-FUEL RATIO CONTROL METHOD

An air-fuel ratio control system () including an air-fuel ratio control section () for controlling the air-fuel ratio λ of an air-fuel mixture, an exhaust gas purifier (); an air-fuel ratio sensor () whose output changes sharply when λ in the exhaust gas changes between rich and lean sides about a stoichiometric air-fuel ratio; a heater (); and a temperature control section (). The air-fuel ratio control section () controls λ based on the output of the air-fuel ratio sensor () using, as a target air-fuel ratio, a predetermined air-fuel ratio such that 0.980≤λ