Apparatus and method for engine exhaust purification

An oxygen storage amount of a catalyst (3) is estimated by a controller (6) in accordance with a sensed air-fuel ratio of an inflowing exhaust gas mixture flowing into the catalyst (3), and a sensed intake air amount of the engine (1), to control the air-fuel ratio of the engine. The estimated oxygen storage amount is corrected to reduce an error in computing the estimated oxygen storage amount when a downstream exhaust condition sensed by an exhaust sensor (5) disposed on the downstream of the catalyst (3) becomes equal to a predetermined threshold, which is modified in accordance with the intake air amount for better emission control performance.