PURPOSE: To make the heat of fuel difficult to dissipate from a combustion cylinder and to prevent deformation of a combustion cylinder, by a method wherein a combustion cylinder is held in a position above a layer filled with fuel by means of an outer combustion cylinder, and the lower part of the combustion cylinder does not make contact with the outer combustion cylinder.

CONSTITUTION: A flange 25, situated above the surface layer of fuel 3, of a combustion cylinder 1 is held by means of a lock piece 26 of an outer combustion cylinder 5, and the combustion cylinder 1 is suspended by means of the lock piece 26. Therefore, heat in the vicinity of a fire grate 2, being a highest temperature part, passes through the upper part of the combustion cylinder 1 and is dissipated from the upper part of the outer combustion cylinder 5. Thus, compared with a case that the high temperature part of a conventional combustion cylinder 1 is held by the lower temperature part of the outer combustion cylinder 5, the heat dissipation amount is decreased, and the temperature of the fuel is easy to maintain even in a low oxygen state. Further, since the combustion cylinder 1 is freely expanded downward from the lock piece 26 serving as a starting point, deformation is prevented from occurring.