To further reduce the generation of NOX increase efficiency, reduce size, and make selection of kind of fuel unnecessary.

At least two annular heat transmission parts 4 and 5 are arranged in a multiple manner and a combustion device 7 to use liquid fuel and gas fuel through selective switch is arranged at one opening end of a first annular heat transfer part on the innermost peripheral side such that the injection direction of flame approximately parallels the axial direction of a first annular heat transfer part 4. The inside of the first heat transfer part 4 forms a first combustion space 11, and a space between the first annular heat transfer part 11 and a second annular heat transfer part 5 positioned at the outside thereof forms a second combustion space 12. The first annular heat transfer part 4 is arranged in the direction of the center thereof in the vicinity of the combustion device 7. A flow gap to communicate together the inside and the outside of the first annular heat transfer part 4 is formed in the peripheral surface of the first annular heat transfer part 4. Flame in the first combustion space 11 is introduced in the second combustion space 12 through the flow gap.