PURPOSE: To prevent the generation of rough idle, an offensive smell, and after fire by providing a final injection quantity operating means which operates a final injection quantity from a basic injection quantity, air-fuel ratio compensating factor, and the learned compensating quantity of air-fuel ratio.

CONSTITUTION: A basic injection quantity operating means operates a basic injection quantity TP in accordance with an operating condition parameter. A feedback air-fuel ratio controlling means compensates the basic injection quantity TP in accordance with an air-fuel ratio compensating factor FAF. A difference operating means operates the difference between an air-fuel ratio compensating factor average value FAVAV and a theoretical value. A final injection quantity operating mans operates a final injection quantity TAV. Thereby, the generation of rough idle, an offensive smell, and after fire caused by the over-rich or the over-lean of air-fuel ratio can be prevented.