Abstract: In an engine exhaust gas purifier having a urea-SCR-catalyst and an injector, the urea-SCR-catalyst is regenerated periodically to remove stack up sulfur caused by the diesel fuel, by stopping supply of urea water, and raising exhaust gas temperature to heat the catalyst. The injector locates in the downstream of cooling wind of a variable speed cooling fan which creates variable cooling wind based on the temperature of the cooling component like radiator, oil cooler, after cooler of the engine. At the regeneration of the urea-SCR-catalyst, controller changes preset fan-speed to higher speed to increase cooling wind to inhibit overheating.