Abstract: A solution composed of a chelating solution and/or an organic acid is applied to radioactive metals to form a radioactive metal solution. The metal solution is increased in its electrical conductivity by addition of an alkaline agent, followed by electrolysis by an electrolyzer to thereby convert metal ions in the metal solution to hydroxides of low solubility and to separate the hydroxides as a filter cake by subsequent filtration. The filtrate is irradiated with ultraviolet rays for further decomposition of remaining organic matter and then filtered by a device using reverse osmosis. The resultant filtrate is recycled as clean water for use for a chelating solution and electrolytic solution, or is discharged. The metal ions precipitated by treatment with the alkaline agent prior to electrolysis as matter of low solubility is separated from the filtrate which is thereafter electrolyzed.