A body to be treated such as a soil or a flying burned ash is introduced into a sealable thermal decomposition furnace (310) and is heated under reduced pressure. A gaseous effluent derived from the body to be treated is treated in a manner wherein formation or resynthesis of dioxin is inhibited. A heating residue of the body to be treated is cooled after it is purged with a gas for displacement which is free of a halogen and has no ability to form an organic halide. By such a treatment, a content of an organic halidc remaining in the heating residue of the body to be treated can be reduced to a very low level.