PURPOSE: To prevent the generation of local heating or other troubles by providing a projection projected from a front wall surface toward a combustion chamber at least over the whole width of a traveling grate on the lower end portion of a linear front wall the terminal end of an ignition arch portion arranged above the inlet of the traveling grate.

CONSTITUTION: A projection 21 projecting on a combustion chamber 2 side is formed over the whole width on the lower end portion of a front wall 3 in order to keep a flame blowing out along an ignition arch portion 7 apart from the surface of the front wall 3. The projection 21 continuously projects from the ignition arch portion 7, makes a detour via a circular surface and is connected to the front wall 3. The length D of the projection 21 measured from the front wall 3 is set to 100-200mm. Thus, the flame from the ignition arch portion 7 is guided toward the combustion chamber 2 to enable the flame to be prevented from rising along the surface of the front wall 3. Accordingly the local heating of a front wall water tube and the trouble of the fusion of attached ash can be prevented from being generated.