In a boiler having a plurality of heat exchanger stages such as a superheater, an evaporator and a preheater, disposed in a combustion gas channel between heat exchangers including an optimum catalytic reaction temperature region are a denitration catalyst layer crossing said channel and a reductant feeding device for adding a reductant such as ammonia gas at a point a little short of catalyst layer. The denitration catalyst layer comprises a number of plate-like catalyst bodies whose plate surfaces extend along the direction of flow of gas, and desirably it is honey-comb-shaped. If the boiler is a waste heat boiler, the denitration catalyst layer will be disposed between the superheater and the evaporator.