NOVELTY - Processing of toxic substance involves planting (S11) of toxic substance absorption plant in contaminated region contaminated with toxic substance, storing (S12) toxic substance in plant, harvesting toxic substance absorption plant, opposingly contacting harvested toxic substance absorption plant with pressurized hot water, performing hydrothermal decomposition process (S13), extracting biomass solid content from which toxic substance is removed from side different from emission side of hot water emission liquid, and processing toxic substance (S14A) from hot water emission liquid.    USE - Processing of toxic substance in soil, polluted liquid and bottom sludge.    ADVANTAGE - The toxic substance in the contaminated soil is efficiently processed.    DETAILED DESCRIPTION - Processing of toxic substance involves carrying out planting of toxic substance absorption plant in contaminated region contaminated with toxic substance, storing toxic substance in toxic substance absorption plant, harvesting toxic substance absorption plant in which toxic substance is stored, and processing the toxic substance contained in the toxic substance absorption plant by opposingly contacting harvested toxic substance absorption plant with pressurized hot water, performing hydrothermal decomposition process, extracting biomass solid content from which toxic substance is removed from side different from emission side of hot water emission liquid while transferring the toxic substance to hot-water side and extracting as hot water emission liquid, performing toxic substance process and processing the toxic substance from the hot water emission liquid.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of the processing of toxic substance. (Drawing includes non-English language text)    Step for planting (S11)    Step for storing toxic substance (S12)    Step for performing hydrothermal decomposition process (S13)    Step for processing toxic substance from hot water emission liquid (S14A)