To effectively decompose and eliminate dioxins left behind in furnace equipment without being discharged to the outside of a system during furnace operation of the furnace equipment, i.e., dioxins adhering to and remaining in the equipment such as a furnace wall, a flue, and a dust collector, and dioxins adhering to or adsorbed to adhering residues, filtration cloths, and waste of decomposed substances in the equipment produced upon cleaning or decomposition of the furnace equipment.

Dioxins deposited or remaining in furnace equipment are processed upon interrupting operation of the furnace equipment by heating the furnace equipment under existence of a dioxin decomposing agent. Waste produced upon cleaning or decomposing the furnace equipment is heated under existence of the dioxins decomposing agent, whereby dioxins adsorbed or deposited on the waste are processed.