PURPOSE: To reduce a CO discharging quantity by setting a first coefficient to a second coefficient which is the average value thereof when the first coefficient which is varied in accordance with the output of an exhaust gas density detector exceeds a defined upper limit value or a defined lower limit value, at the time of the low temperature of an engine.

CONSTITUTION: At the time of the feedback control of air-fuel ratio of an engine 1, an ECU 5 controls the air-fuel ratio using a first coefficient which is varied in accordance with the output of an exhaust gas density detector 15 provided on the exhaust system 13 of an engine, while calculated a second coefficient which is the average value of the first coefficient. When the first coefficient exceeds a defined upper limit value or a defined lower limit value at the time of low temperature of an engine detected by a water temperature sensor 10, the first coefficient is set to a second coefficient to continue the air-fuel ratio feedback control. Also, the upper limit value is set larger whereas the lower limit value is set smaller as the temperature of the engine is lower. Thereby, the discharge of CO at the time of low temperature can be reduced.