PURPOSE: To lengthen the mixing distance of the mixture so as to improve the mixing condition by first injecting fuel and air into the outer space between the mixing chamber and inner shell and then introducing said mixture in the outer space into the inner space of the inner shell through the openings in the inner shell.

CONSTITUTION: Part of combustion air supplied into a space 36 between the outer shell 34 and combustion chamber 32 and mixing chamber 33 passes through a gap between a mixing pipe 49 and the end 52a of a fuel injection nozzle 52 to be led to a space 50 between the mixing chamber 33 and inner shell 46. At the same time, fuel gas is injected into the space 50 from the fuel injection nozzle 52, which is mixed with air as it spirals around the outer surface of the inner shell 46. The mixture is introduced from the space 50 to the inner space 47 of the inner shell 46, and then is injected through mixture injection holes 48 into the combustion chamber 32, where it is burnt. Therefore, even though the axial length of the mixing chamber 33 is short, the mixing distance for the mixture can be made long enough. As a result, the overall size can be made compact.