PURPOSE: To enhance transient responsiveness to fluctuations in load and in engine speed by controlling an air bleed control valve on the basis of the amount of feedback control that is obtained by the addition of a basic controlled value corresponding to each of the operating conditions of the engine and a correction that is based on a detection signal of an O2 sensor.

CONSTITUTION: In air-fuel ratio control performed by an ECU 11, the engine speed Ne and throttle opening To are read and then the basic opening of an air bleed control valve 12 in an operating condition is read from the data Ne, To by means of a control map. Next, the amount of feedback control of the valve 12 is computed from the sum of the basic opening and a feedback correction that is based on an output signal of an O2 sensor 10, thereby performing feedback control of the opening of the valve 12. Therefore, even when loads and the like fluctuate, the valve opening required to attain a stoichiometric air-fuel ratio under each operating condition can be attained quickly, and the stoichiometric air-fuel ratio can be attained with good responsiveness.