To hold airtightness in a system by arranging a scraper conveyor from a lower part of a slag dumping port to an upper part of a slag hole in a casing, and cooling a bottom plate or both sides of an inside between the bottom plate and the casing from outside, thereby finely granulating slag and enhancing a slag discharging capacity.

A slag dumping side end 2a is arranged at one end of a casing 2, and a slag discharging side end 3b is arranged at the other end. A cooler 4 is arranged from an outside lower surface of the casing 2 to both side faces from the end 2a to the end 3b. A scraper conveyor 11 is arranged from the end 2a to the end 2b in the casing 2. The slag scattered on a bottom plate 13 is captured by a scraper moving from the end 2a toward the end 2b on the plate 13, dumped from a slag hole 1f on a slag conveyor 5, and the plate 13 and the casing 2 are cooled by the cooler 4.