To prolong the life of a refractory brick.

A stamp material 19 made of a silicon carbide material with high heat conductivity is interposed between an iron shell 12 and a refractory brick layer 18 inside the iron shell 12 to form a furnace wall 8a. The refractory brick layer 18 is a dual structure constituted of a ceiling refractory brick 20 and a lining refractory brick 21 with high heat conductivity. Insulation sheets 22 are interposed between respective layers of the iron shell 12, the stamp material 19, the lining refractory brick 21 layer and the ceiling refractory brick 20 layer. Thermal expansion of the refractory bricks 20, 21 at melting and processing of ash inside the furnace is absorbed by the stamp material 19. At the same time, by transferring water cooling effect of the iron shall 12 outer surface to the ceiling refractory brick 20 via the stamp material 19 with high heat conductivity and the lining refractory brick 21, cooling effect of the ceiling refractory brick 20 is improved and wearing is restrained.