PURPOSE: To reduce required energy by treating valuables conc. gas obtained by treating a dilute solution with an osmotic evaporation method with a gas separation membrane.

CONSTITUTION: A solution is sent to the supply part 21 of an osmotic evaporation separation membrane apparatus 2 by a pump 1 while preheated by a preheater 3 and the liquid penetrating a membrane is evaporated under the reduced pressure due to a vacuum pump 4. A gaseous mixture is sent to a gas separation membrane apparatus 5 and non-valuable gas separated by penetration under the reduced pressure due to a vacuum pump 6 by the permselectivity of a gas separation membrane and liquefied by a condenser 8 while the condensed liquid is discharged along with the treated solution passed through the supply part of the osmotic evaporation separation membrane apparatus 2 in a non-penetration state. During a period when the gaseous mixture flows toward an outlet 510 through the supply part 51 of the gas separation membrane apparatus 5, the concn. of valuables gas in enhanced further the this penetrating gas is liquefied by a condenser 7 to recover valuables in a liquid phase. By this method, valuables can be recovered with low energy.