To enable dioxins in soil to be efficiently decomposed in decomposing a halogenated organic compound by thermally reacting the halogenated organic compound with a polyamine.

In decomposing dioxins contained in the soil of a refuse incineration site or its surroundings, dioxins or halogenated organic compound which is dioxins-containing substance is thermally reacted with a polyamine. A polyamine is a polymer having many nitrogen-containing groups such as amino or imino groups per molecule, and the reaction of the nitrogen groups of the polyamine with a halogen of the halogenated organic compound, such as chlorine of dioxins, causes the decomposition. Polyvinylformamide, polyvinylacetamide or the like which generates a polyamine when decomposed may be used as a polyamine, and so may a copolymer or copolycondensate of a polyamine with another monomer.