PURPOSE: To discharge adequately an intruded light gas and to improve the recovering efficiency of gaseous UF6 by determining the total pressure of the gas to be treated and the partial pressure of the light gas, comparing the same with set values and controlling gas flow passages in accordance with the result thereof.

CONSTITUTION: The gaseous UF6 supplied from a tank 1 is increased in pressure by compressors 3, 4 and is condensed and recovered into the 1st and 2nd recovering cylinders 16a, 16b through a pressure regulating tank 5. The total pressure P0 of the gas is determined by a pressure gauge 6 provided on the upper stream side of the recovering cylinders in the above-mentioned recovering system and further the partial pressure Pk is determined by using the value obtd. by a γ ray detector 9 and a thermometer 11. (I) The UF6 is recovered into the cylinder 16a if P0<Pos (Pos is the 1st set value) (a valve 15a is open, 15b, 17a, 17b are closed) and the recovery of the UF6 is changed over to the cylinder 16b (the valve 15a is closed and 15b is open) if P0>Pos in the case of Pk<Ps (Ps in the 2nd set value). On the other hand, the light gas accumulated from the cylinder 16a (16b) is fed together with the UF6 to a discharge device 18 [the valve 16a (16b) is closed and 17a (17b) is open] in the case of Pk>Ps.