To regenerate a catalyst in which a masking layer is formed on the surface simply by a method in which when the catalyst in which catalyst components are supported on a carrier is regenerated, the raw material slurry of the catalyst components and the carrier is applied on the surface of the catalyst and dried or baked, and a coating film is formed.

In order to decompose/remove harmful substances in exhaust gas, NOx, ammonia, and others are decomposed into nitrogen gas stream and others by the reaction in which the exhaust gas is brought into contact with a catalyst in which catalyst components are supported on a porous carrier. By continuing the decomposition treatment, trace amounts of components such as As in the exhaust gas are deposited gradually on the surface of the catalyst to form a masking layer. When the functional deterioration of the catalyst takes place, the catalyst is immersed in the raw material slurry of the catalyst components and the carrier to apply the slurry on the surface of the catalyst and heated in a heating furnace to be dried or baked. In this way, the catalyst can be regenerated by forming a new coating film to be a catalyst layer on the surface of the masking layer.