PURPOSE: To prevent an air-fuel ratio from being controlled to an erroneous value by a method wherein table characteristics according to which a relation between an O2 sensor output and an air-fuel ratio is determined are changed according to an operation condition.

CONSTITUTION: The characteristics of an O2 sensor output to an air-fuel ratio are brought into a table classified by each of areas, into which an operation region is divided, and prestored in a memory means 21. When it is decided by a deciding means 22 to which area of the areas an operation condition belongs, a table corresponding to the decided area is selected from a plurality of tables, stored at the memory means 21, by a selecting means 23. By causing a reference means 25 to refer to the selected table, an air-fuel ratio is determined from an O2 sensor output. Based on the air-fuel ratio, feedback control of an air-fuel ratio is performed by a control means 26.