Exhaust gas cleaning system and SOx poisoning recovery method for internal combustion engine

An exhaust gas cleaning system comprises sulfur oxide (SO x) occlusion amount estimation section (20a), exhaust air-fuel ratio control section (20b), sulfur oxide poisoning recovery section (20c) and hydrocarbon concentration detector (20d). When the sulfur oxide occluded in nitrogen oxide (NO x) catalyst (16) is released, the air-fuel ratio of exhaust gas flowing into catalyst is controlled at richer side with decreased amount of sulfur oxide occlusion. An independent claim is included for sulfur oxide poisoning recovery method.