AIR-FUEL RATIO CONTROL DEVICE FOR ENGINE AND AIR-FUEL RATIO CONTROL METHOD FOR ENGINE

PROBLEM TO BE SOLVED: To propose an air-fuel ratio control device for an engine and an air-fuel ratio control method for the engine, which can stabilize idling, even for the engine for which activity of an oxygen concentration sensor can be achieved with difficulty in an idling state.SOLUTION: The air-fuel ratio control device 31 of the engine 18 includes: a throttle sensor 33 detecting whether a throttle valve 32 is in fully closed state or not; an engine speed sensor 35; an idling determination part 36 determining from the fully closed state of the throttle valve 32 and engine speed of the engine 18 whether the engine 18 is in an idling state or not; the oxygen concentration sensor 37 measuring oxygen concentration of exhaust gas of the engine 18; a heater 38 heating the oxygen concentration sensor 37; and an ECU 43 maintaining a central value of a collection factor for controlling an fuel supply amount when change of the correction factor in time series converges within a predetermined range in the idling state, and continuously or intermittently stopping power supply to the heater 38.