PURPOSE: To safely recover impurities and to reduce the size of a recovering vessel by introducing radioactivated gaseous impurities captured in the body of a purifier into a degassing pot, mixing the impurities with air, carrying out oxidation treatment and liquefaction by cooling, and recovering the resulting liq.

CONSTITUTION: A liq. metal such as liq. sodium contg. impurities such as gaseous hydrogen in a fast breeder reactor is circulated through the body 1 of a purifier, and by cooling the vessel 2 of the body 1 with a cooler 3, the impurities are adsorbed on the net 4. A heater 5, an exhaust fan 7, a blower 17 and a cooler 20 are actuated to introduce the impurities captured by the net 4 into a degassing pot 6 in the gaseous state, and the impurities are fed to an oxidation treating apparatus 18. On the way to the apparatus 18, the gaseous mixture of the gaseous impurities is mixed with 4vol% air from the blower 17. The reaction of the gaseous impurities with oxygen in the air is accelerated in the presence of catalyst 19 in the apparatus 18, water produced by cooling with the cooler 20 is allowed to flow into a recovering vessel 22, and the unreacted excess gas is fed to a waste gas treating apparatus through an exhaust cylinder 23.