By reaction of halogenated organic compounds with at least one amino acid compound selected from a group consisting of amino acid, polyamino acid, amino acid salts and polyamino acid salts under heating, halogenated organic compounds such as dioxins contained in soil can be easily and effectively decomposed. By adding and mixing the amino acid compound to the halogenated organic compounds, followed by adding and mixing of a water-absorbing agent thereto and subsequent heating, an effective decomposition treatment can be carried out with excellent operation properties, preventing lumping and deposition of the treated material. A method for decomposing halogenated organic compounds in which the amino acid compound is added and mixed with solid material such as soil contaminated with halogenated organic compounds, followed by adding and mixing of a water-absorbing agent and subsequent heating of resultant mixture to decompose halogenated organic compounds. They are mixed in a mixer which contains mixing media in a drum rotating around a horizontal axis while simultaneously vibrating, an uniform mixing is achieved by mutual collision of the solid material and the mixing media, enabling an effective decomposition treatment.