PURPOSE: To prevent fluctuation of the controlled air-fuel ratio due to misfiring of an engine, by detecting misfiring of the engine, and changing the feedback control of air-fuel ratio using a means for detecting the components of exhaust gas when misfiring of engine is detected.

CONSTITUTION: An apparatus of this invention, intended to control the air fuel ratio of mixture to be supplied to an engine according to the output of a means for detecting the density of components contained in the exhaust gas of engine, is designed to detect misfiring of the engine and to stop the air-fuel ratio control based on the output of the means for detecting the components of exhaust gas or otherwise to change the amied value of air-fuel ratio when misfiring of the engine is detected. By employing such an arrangement, it is enabled to prevent fluctuation of air-fuel ratio and to keep the air-fuel ratio stable.