PURPOSE: To prevent an over-heating of a secondary air pipe and improve its durability by a method wherein the secondary air pipe is arranged above a flame port plate, its opening end is connected to a side wall of the secondary air chamber and a water pipe of a heat exchanger is brought into contact with an inner wall surface of the secondary air chamber.

CONSTITUTION: A secondary air pipe 18 is arranged above a flame port plate 14, both end ports of the secondary air pipe 18 are opened at a secondary air chamber 16, respectively and at the same time the opening ends of the secondary air pipe 18 is connected to the side wall of the secondary air chamber 16 and a water pipe 19 of a heat exchanger 3 is fitted to the secondary air chamber 16 and the water pipe 19 is brought into contact with an inner wall surface of the secondary air chamber 16. In this way, the secondary air chamber 16 is cooled by water flowing in the heat exchanger 3 and subsequently to the secondary air pipe 18 can be cooled.