Air-fuel ratio control apparatus and method of internal combustion engine

An air-fuel ratio control method of an internal combustion engine having an air-fuel ratio sensor capable of detecting a stoichiometric air-fuel ratio in an exhaust gas of the engine, comprising:

• determining whether an engine is operated in a rich operational region where fuel supply to the engine is increased, and
• performing an air-fuel ratio feedback control for bringing an air-fuel ratio of the engine toward a target air-fuel ratio on the basis of an output from the air-fuel ratio sensor, in which the target air-fuel ratio is a rich air-fuel ratio when the engine is in the rich operational region, the air-fuel ratio feedback control being performed with a feedback coefficient selectively bringing the air-fuel ratio toward the target air-fuel ratio and performed by selectively limiting the feedback coefficient at a limit value, the limit value used in the rich operational region being determined such that the feedback coefficient is generally limited as compared to a limit value used in an operational region other than the rich operational region.