The membrane is a permselective, composite membrane composed of a porous support and a nonporous, active membrane of a synthetic resin formed on the porous support, or is a permeable membrane having such characteristics that the nitrogen gas permeation rate at 30??? C. is in the range from 7???10-4 to 2???102 Nm3 m2 ???h???atom and that the amount of permeated steam is 100 g/m2 ???h or less when 2