PURPOSE: To improve combustion efficiency by enabling heated fresh air to be supplied as secondary combustion air, by connecting one side of an air slit provided between a furnace wall and an external housing with a combustion chamber.

CONSTITUTION: An air slit 5 is provided between a furnace wall forming a combustion chamber 1 and an external housing 3, and the air slit 5 and combustion chamber 1 are connected through a heated air leading-in openeing 7 with each other. By constituting in this manner, fresh air is induced from an air leading- in opening 6 of the air slit 5 apart from primary air entering from a leading- in opening 10 for the primary air through negative pressure within the combustion chamber 1, and this air is heated while it is running down through the air slit 5, which is supplied into a lower combustion chamber 1b as secondary air from the heated air leading-in opening 7.