PURPOSE: To form an inexpensive and highly heat resistant separating membrane by forming a membrane body by the use of a dope prepd. by dissolving polyallylate resin in N-methyl-2-pyrrolidone.

CONSTITUTION: A water soluble polymer such as polyethylene glycol and an org. solvent are contained in accordance with need in a dope prepd. by contg. 5W 25wt% polyallylate resin at ≤100°C, whereby the rate of permeation of the resultant separating membrane is increased. This dope is cast with a doctor knife on backing and this backing is dipped in dipping liquid which is nonsolvent to allow the dope to gel, or it is dipped in the dipping liquid after part of the solvent in the formed film is allowed to evaporate, whereby the dope is gelled. Nonwoven fabrics and the like are used for the substrate, and water, alcohol, etc. are used for the dipping liquid. In this way, the separating membrane with ≤100μ thickness, usually about 20W80μ consisting of the polyallylate resin is obtained. If necessary, this is blanked to desired shapes such as circular or doughnut shapes.