PURPOSE: To stabilize the operation of a vertical furnace by extracting materials containing a plenty of heavy substances such as iron or the like of residues exhausted from the furnace, again charging the substance into the furnace and reacting a solid waste layer while compressing the layer to thereby prevent blow-by and hanging phenomena.

CONSTITUTION: Solid waste 7 and auxiliary fuel 8 or the like such as coke or the like are charged from a hopper 2 into a vertical furnace 1, reacted or incinerated in the furnace, and the residues thereof are exhausted from the lower outlet 3 thereof. The residues are readily separated into substance 9 containing a plenty of irons and other substances 10 by means of their density and magnetic differences. The necessary amount 11 of the substance 9 is roughly crushed in case of large lump, or granulated in case of small particles, and charged in the same manner as the solid waste or auxiliary fuel or the like from the hopper 2. Thus, the same substance layers 11a are alternatively formed with the solid waste layers 7a in several stacks in the furnace so that the solid waste is compressed by the layers 11a to thereby prevent abnormal phenomena such as blow-by and hanging phenomena, etc. so as to stabilize the operation of the furnace.