A catalyst recovery method is capable of maintaining NOx purification performance by efficiently releasing SOx occluded in a NOx catalyst in an exhaust system of an internal combustion engine. The air fuel ratio of exhaust gas in a fuel supply period (.DELTA.t1) is made further richer (C2) than a predetermined rich air fuel ratio (C0) with increasing length of a set stop period (.DELTA.t2). Provided within a stop period (.DELTA.t2) is an excessively lean period (.DELTA.t3) in which the air fuel ratio of the exhaust gas becomes higher (C3) with increasing length of the stop period (.DELTA.t2), whereby an amount of reducing agent to be added is increased, thus making it possible to ensure a sufficient amount of SOx released from the NOx catalyst. As a result, the length of the stop period (.DELTA.t2') is shortened, so the SOx occluded in the NOx catalyst can be efficiently released.