PURPOSE: To improve thermal efficiency upon high load of a boiler and prevent the deterioration of catalyst upon low load of the same by providing a tube type air preheater on the half way of an exhaust gas duct.

CONSTITUTION: The rear heat transfer section 24 of fire furnace of a boiler is constituted so that the temperature of exhaust gas 11 at the inlet port of a denitrating device 15 upon the low load of a boiler, in which a bypass damper 22 is opened, is kept higher than the lower limit temperature of the inlet port of a denitrating device 15. For this purpose, the temperature of exhaust gas 11 before introducing it into the denitrating device 15 can be reduced by supplying air to a tube type air preheater 29 upon the high load of the boiler even if the temperature of the exhaust gas 11 at the inlet port of the denitrating device 15 is risen. Accordingly, the temperature of the exhaust gas at the outlet port of a downstream side air preheater 16 is kept lower than a predetermined temperature whereby the thermal efficiency of the whole of the boiler can be maintained in high. According to this method, thermal efficiency upon the high load of the boiler is improved and the deterioration of catalyst due to the reduction of the temperature of exhaust gas 11 upon low load of the boiler can be prevented.