PURPOSE: To inexpensively separate a volatile mixture by separating a mixed vapor fed from a distillation column by a separating membrane into a low boiling point (B.P.) material and a high boiling point material, and returning the low B.P. material to a concentrating stage and the high B.P. material to a recovering stage.

CONSTITUTION: The volatile mixture is fed from a line A to an intermediate stage 2 of the distillation column 1 and indirectly heated; a part of it is vaporized; and the remainder is flowed down as a mixed liquid in the distillation column 1. The vaporized mixed vapor is withdrawn through the line B; after heated up by a superheater 9, the high B.P. material is selectively permeated to the secondary side 4b through the gas separating membrane 4; a non- permeated low B.P. material is returned to the concentrating stage 2 of the distillating column 1; the high B.P. material is transmitted to a cooler 5 through the line D and after condensation, it is returned from the line E to the recovery stage 2" of the distillation column 1 through a vacuum tank 6.