An object to be decontaminated contaminated with radioactive material, e.g., contaminated soil or water, is introduced into eluting solvent and dissolved, and the radioactive material is separated from the object to be contaminated by elution of the radioactive material into the eluting solvent. The eluting solvent containing the radioactive materials dissolved therein and the object to be decontaminated are separated into solid and liquid. The soil after solid-liquid separation and from which the radioactive material is removed is collected, and the eluting solvent after solid-liquid separation and a separated liquid containing contaminated water are introduced into an electrolysis tank and electrolyzed. Metal ions such as those of the radioactive materials are deposited on the cathode in the electrolysis tank. Hydrogen containing tritium generated in electrolysis is collected in the electrolysis tank. The hydrogen is moved to the outside of the electrolysis tank and trapped.