Abstract: A boiler apparatus having a furnace and a plurality of heat exchanger stages disposed in a channel for the combustion gas discharged from the furnace. The boiler apparatus is equipped with a denitrator which utilizes a catalyst and which is disposed in the optimum reaction temperature region for the catalyst in the channel. In order to control the temperature of the combustion gas in the optimum reaction temperature region, this region is adapted to communicate with a high temperature gas source or a low temperature gas source through a control valve. Desirably, the portion of the combustion gas channel which includes the optimum reaction temperature region is divided into a plurality of systems and dampers whose opening and closing are controlled are provided upstream and downstream of the region.