PROBLEM TO BE SOLVED: To provide a treatment method of harmful substances which can efficiently recover harmful substances from plants absorbed the harmful substances to process while clarifying contaminated soil.SOLUTION: There is provided the treatment method of harmful substances, wherein radioactive substance absorbing plants are cropped in a contaminated area contaminated with radioactive substances, the radioactive substances are accumulated in the radioactive substance absorbing plants, the radioactive substance absorbing plants containing the radioactive substances are harvested, and the radioactive substances contained in the radioactive substance absorbing plants are processed. The treatment method of harmful substances includes: a hydrothermal decomposition treatment step (S13) for counter-contacting the harvested radioactive substance absorbing plants with pressurized hot water, shifting the radioactive substances to the hot water to extract as hot water discharge liquid while extracting a biomass solid content in which the radioactive substances are removed from a side different from the side discharged the hot water discharge liquid； and a radioactive substance treatment step (S14) for treating the radioactive substances from the hot water discharge liquid.【課題】汚染土壌の浄化を図りつつ、有害物質を吸収した植物から有害物質を効率良く回収し、処理することができる有害物質の処理方法を提供する。【解決手段】本発明に係る有害物質の処理方法は、放射性物質で汚染された汚染領域に放射性物質吸収植物を作付し、放射性物質吸収植物に放射性物質を蓄積させ、放射性物質を蓄積した放射性物質吸収植物を収穫し、放射性物質吸収植物中に含まれる放射性物質を処理する有害物質の処理方法であって、収穫した放射性物質吸収植物を加圧熱水と対向接触させて、放射性物質を熱水側に移行させて熱水排出液として抜き出すと共に、放射性物質が除去されたバイオマス固形分を熱水排出液の排出側とは異なる側から抜き出す水熱分解処理工程（Ｓ１３）と、熱水排出液から放射性物質を処理する放射性物質処理工程（Ｓ１４）とを含む。【選択図】図１
