PURPOSE: To prevent the corrosion of the downstream equipment by providing a heat exchanger between an economizer at the inlet of a boiler and a denitrator to cool the waste combustion gas and suppressing the SO2 oxidation of a denitration catalyst.

CONSTITUTION: A first air preheater 4 is provided as a heat exchanger between the economizer 1 of a boiler and a denitrator 2, and a second air preheater 5 is arranged on the downstream side of the denitrator 2. Since the heat exchanger is furnished on the upstream side of the denitrator 2, the gas temp. at the denitrator 2 inlet is lowered and maintained in an appropriate range where the SO2 oxidation rate of the catalyst is not increased and SO3 is not formed. A regeneration-type rotary air preheater, a steel-tube air preheater, etc., to exchange heat between the boiler waste gas and combustion air and further a device for generating auxiliary steam with the boiler waste gas are used as the heat exchanger, and any of type or cooling medium is used. The low-temp. corrosion of the equipment arranged on the downstream side of the denitrator is prevented.