PURPOSE: To separate highly pure olefins from a gaseous feed stream under the conditions of resistance to membrane dryness and long membrane life by using a porous support structure and a liquid membrane containing an aqueous liquid containing a metal salt facilitator capable of coordinating with olefin gases and an alkyl carbonate co-solvent.

CONSTITUTION: In the porous support structure 10 (e.g. hollow fiber), a facilitated liquid membrane containing an aqueous liquid containing a metal salt facilitator (e.g. AgBF4) capable of coordinating with olefin gases and an alkyl carbonate co-solvent (e.g. propylene carbonate) are formed. Into one of the facilitated liquid membranes, a gaseous feed stream such as a paraffin gas is sent and olefins are collected on the other side of the membrane. As a result, highly pure olefins with resistance to membrane dryness and long life can be obtained.