EXHAUST GAS CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINE AND EXHAUST GAS CONTROL METHOD FOR THE SAME

An exhaust gas control apparatus for an internal combustion engine includes: a catalyst which is capable of storing oxygen; a downstream air-fuel ratio sensor that detects the air-fuel ratio of outgoing exhaust gas flowing out of the catalyst; an air-fuel ratio control unit that controls the air-fuel ratio of incoming exhaust gas flowing into the catalyst; and a catalyst state estimation unit that estimates the activity of the catalyst. The air-fuel ratio control unit controls the air-fuel ratio of the incoming exhaust gas so that the air-fuel ratio of the outgoing exhaust gas detected by the downstream air-fuel ratio sensor is maintained at a target air-fuel ratio. The air-fuel ratio control unit sets the target air-fuel ratio to a richer value when the activity of the catalyst is equal to or higher than a predetermined value than when the activity of the catalyst is lower than the predetermined value.