PROBLEM TO BE SOLVED: To provide a treatment device that removes chloride, at a low cost and a high level, from radioactive contaminated water comprising radioactive material and chloride.SOLUTION: The treatment device comprises lead addition means 2 for adding a lead compound to radioactive contaminated water PW comprising chloride to separate it into first contaminated water and lead chloride precipitate, and silver addition means 3 for adding a silver compound to the first contaminated water discharged from the lead addition means 2 to separate it into second contaminated water and silver chloride precipitate.