This is a method for regenerating the NOx catalyst in a NOx purifying system provided in the exhaust passage with a direct reduction type NOx catalyst which directly decomposes the NOx during lean-condition operation and is regenerated at rich-condition operation, and the method prohibiting the rich-condition control when the temperature detected by a catalyst temperature detecting means is within the predetermined temperature range.Thus, when the exhaust gas are temporarily made rich-condition for catalyst regeneration which means recovering the NOx purifying ability of the direct reduction type NOx catalyst arranged in the exhaust passage of the engine, the NOx is prohibited from being discharged into the atmospheric air, and also the purifying ability can surely be recovered.