PURPOSE: To reduce NOx emission without remodeling the combustion mechanism and the combustion control part on a large scale by providing a feedwater branch pipe and a water tank equipped with a steam-evolving mechanism, said branch pipe branching off from the feedwater pipe which leads to a heat exchanger and having the forward end open inside said water tank and said water tank having in its body an opening to a combustion-air chamber.

CONSTITUTION: A branch pipe 9 branches off from a feedwater pipe 7. The branch pipe 9 is provided with a water-flow control valve 10, has the forward end inserted into a water tank 11 provided in the lower part of a combustion-air chamber (a) and has the forward end open inside the water tank 11. The water tank 11 is equipped at its bottom with a steam- evolving mechanism 12 and has in its body an opening 13 to the combustion-air chamber (a). The supply of gas to a gas burner 1 is turned on and an electric fan 4 is driven for combustion and the steam-evolving mechanism 12 is driven, then the water in the water tank 11 thereby vaporizes into steam, which enters the combustion-air chamber (a) through the opening 13. This introduction of steam into the flow of primary combustion air and secondary combustion air causes the heat capacity of the combustion air to increase and hence effects absorption of heat from the flames and the surroundings so that with the combustion reaction retarded to a moderate degree a decrease can be achieved in the amount of NOx discharged.