[en] A large amount of low-level radioactive waste metal will be generated from a nuclear power plant decommissioning. Disposal of the radioactive waste must be a burden to the disposal facility, therefore reducing the disposal volume must be important issue to solve. Toward minimizing the volume of radioactive waste, the authors developed a new technique to separate cobalt and nickel elements (not nuclide) from radioactive metal, especially for activated metal, thereby to lower radioactivity concentration of the metal. Screening tests and demonstration tests were implemented. In the screening tests, three kinds of processes were compared on the performance in small-scale apparatus and the Oxygen Sparging Process was seen to have most feasible performance. In the demonstration tests, the prototype apparatus of the Oxygen Sparging Process demonstrated that it could separate cobalt from carbon steel with a decontamination factor of 100 and with an iron recovery ratio of 60% in proper process conditions. (author)