PURPOSE: To reduce the radioactive secondary waste generation by oxidizing the trivallent Ce in decontamination waste liquid to quadrivallent Ce with electrolytic oxidation, separating the quadrivallent Ce and acid from the processed decontamination waste liquid and reusing it.

CONSTITUTION: Decontamination waste liquid 11 is oxidized by electrolysis 12 to raise the concentration of Ce4+ in the decontamination waste liquid and the waste liquid and dialysis liquid 14 are passed by water through a negative ion exchanger film 15. Then, the acid in the waste liquid 11 and Ce4+ having formed complex negative ion move through the exchanger film to the dialysis liquid and are recovered as recovery solution 16. The recovery solution after controlling 17 concentration is reused as decontamination liquid. On the other hand, a little Ce4+ remains in the processed waste liquid 19 eliminated of acid and Ce4+ and therefore, by adding hydrogen peroxide to reduce in process 20 to be chemically stable Ce4+. The reduction processed 18 process waste liquid 19 is neutralized in process 21 with sodiumhydrate solution and dried and solidified 22 sludge and sodium salt are stored as radioactive waste.