To suppress the occurrence of hazardous materials, such as the dioxin, and, at the same time, to prevent secondary pollution caused by eluted metals, by integrally forming a gasifying melting furnace type vertical combustor and the boiler function section of a secondary combustion chamber.

RDF (solidified fuel) starts to make thermal decomposition due a hot blast coming up from the lower part in a preheating and gasifying zone 6, descends to a burning and melting zone 7 where the RCF reaches the highest burning temperature region due to blown-in high-concentration oxygen 8, and melts 9. The pyrolysis gas of the RDF is guided to a secondary combustion chamber 14 in an upper section and completely burned together with blown-in combustion air 5. The waste gas from the chamber 14 is cooled in such a way that the sensible heat of the gas is recovered by means of a superheater 18, a feed water heater 19, and a combustion air preheater 20 arranged in a duct. The superheated steam of the superheater 18 is further superheated by means of a superheater in the combustion chamber 14. The waste gas 21 from the chamber 14 is discharged to the atmosphere. The high-pressure superheated steam produced in a boiler is used for driving a power generating turbine and, after being condensed, used as boiler water.