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

The oxygen storage amount of a three-way catalyst is estimated based on an upstream-side exhaust air-fuel ratio, and a target air-fuel ratio is controlled such that the estimated oxygen storage amount matches a predetermined target oxygen storage amount. When a downstream-side exhaust air-fuel ratio detected by a downstream-side air-fuel ratio sensor is equal to or less than a threshold value corresponding to a first oxygen storage amount, the estimated oxygen storage amount is reset to the value of first oxygen storage amount. When downstream-side exhaust air-fuel ratio is equal to or greater than a threshold value corresponding to a second oxygen storage amount, the estimated oxygen storage amount is reset to the value of second oxygen storage amount. The target oxygen storage amount is set to be closer to first oxygen storage amount than the median of first oxygen storage amount and second oxygen storage amount.