PURPOSE: To prevent the occurrence of deformation and curving, by a method wherein a plurality of longitudinal ribs are formed on the surface part of a flat combustion chamber, and in a heating-to-redness part at the upper part of the front of the combustion chamber, a portion between the ribs forms a curved part which is protruded longitudinally in succession in the same direction as that of the rib.

CONSTITUTION: Although the wall surface of a combustion chamber 15, heated by a combustion heat, is thermally expanded, an expansion amount is absorbed by means of elasticity of the rib 19, and the width of an outer shape of the combustion chamber 15 is held about at the same size only resulting in change in the groove width of the rib 19. meanwhile, in a heating-to-redness part 17, by curving a surface part between the ribs 19 in the same direction as that of the rib 19, mechanical strength is also increased, an effect of absorbing thermal expansion is also produced, and besides, since the width of the interior of the combustion chamber 15 is inhibited against wide change, during combustion, the interior of the combustion chamber 15 is maintained about in the same shape as a shape during cooling. This constitution maintains uniform complete combustion of the combustion chamber, and also increases a radiant heat release from the heating-to-redness part.