An exhaust gas decontamination system and a method of exhaust gas decontamination wherein in the regeneration of NO.sub.x occlusion reduction type catalyst with respect to a lean-burn engine or diesel engine, even in the instance of rich burning accompanied by EGR the NO.sub.x released from the NO.sub.x occlusion reduction type catalyst can satisfactorily be reduced and purged to thereby exhibit a high NO.sub.x removal ratio. The exhaust gas decontamination system (1) provided with the NO.sub.x occlusion reduction type catalyst (30) includes a control unit (C1) comprising a.norrnal control operation feature (C10), a regeneration control initiation judging feature (C20), a catalyst activation control-operation feature (C30) and a rich-burn control operation feature (C40) capable of lowering the concentration of oxygen in the exhaust gas so as to realize such a constitution that the catalyst is carried out just before rich burning accompanied by EGR.