PURPOSE: To effectively reduce the gas temperature at the inlet of an evaporating water tube group without increasing the size of a furnace body.

CONSTITUTION: A gas temperature regulating economizer 7 is installed between a heater 6 and a screen tube 8 at the rear of the heater. Feed water fed from a boiler feed water pump 15 is heated with a low-temperature economizer 13 and then led to the gas temperature regulating economizer 7 to cool a combustion gas at the inlet of an evaporating water tube group 9 below or in the vicinity of the eutectic point. The combustion gas cooled with the evaporating water tube group 9 is further fed to a high-temperature economizer 14. The gas temperature regulating economizer 7 has a relatively coarse tube pitch in the same manner as that of a superheater 6 installed above a nose baffle 4 and is hung from the upper part to ensure a passage of dust on the nose baffle 4. Thereby, the combustion gas from a superheater 6 is cooled.