PURPOSE: To intend improvement of separation effectiveness by separating radioactive nuclides through oxidization process of low level radioactive waste liquid generated from a reprocessing plant and by precipitating remaining radioactive nuclides by adding a coprecipitation agent containing an La3+, to liquid of oxidization process.

CONSTITUTION: Low level radioactive waste liquid 1 generated from a reprocessing plant, is subject to an oxidization process 8 by ozone from an oxidization device in which an ozone generator is built, and therewith radioactive nuclides contained in the waste liquid 1 are separated. Gas generated during the process is discharged 10 in a form of gas after being subject to a gas processing 9. Subsequently, a coprecipitation agent 11 containing an La3+ is added to liquid of oxidation process and remaining radioactive nuclides are subject to nuclide separation to produce precipitates thereof. Supernatant liquid 13, after being filtrated 14, is subject to vaporization process 2, neutralization process 3 and solidification process 6, and stored 7, in the same way as in conventional processing procedures.