In a NOx removal catalyst used for removing nitrogen oxides in flue gas, when a silica (Si) component as an inhibitor that causes an increase in a SO2 oxidation rate accumulates on a surface of the catalyst, the silica component accumulating on the surface of the catalyst is dissolved, thereby regenerating the catalyst. Accordingly, the inhibitor such as the silica component covering the surface of the NOx removal catalyst can be removed, thereby enabling to provide a catalyst without having an increase in the SO2 oxidation rate of the regenerated NOx removal catalyst.