PURPOSE: To concentrate, separate and dehydrate organic compd. with easy operation at low cost by using a hydrophilic membrane and subjecting an organic compd./water-base mixture to membrane separation treatment with a vapor permeation method.

CONSTITUTION: For example, an ethanol/water-base mixture is subjected to membrane separation treatment with a vapor permeation method by using a hydrophilic membrane such as a cellophane membrane and feeding the mixture to the primary side of the membrane in a vapor state. By such a way, swelling of the membrane is inhibited and permselectivities of the membrane can be raised and water is allowed to selectively permeate the membrane on the secondary side and thereby organic compd. such as ethanol can be concentrated, separated and furthermore dehydrated.