PURPOSE: To enhance solvent resistance and mechanical strength by forming a discontinuous sepn. membrane having micropores of an average pore diameter of 0.1 to 10 μm and ≥1 linear alternating polymer of carbon monoxide and an ethylenically unsatd. compd.

CONSTITUTION: A soln. is prepd. by mixing ≥1 linear alternating polymer of carbon monoxide and at least one ethylenically unsatd. compd. After the soln. is cast onto a substrate, the solvent is at least partly removed, by which a discontinuous sepn. membrane having the micropores of the average pore diameter of 0.1 to 10 μm is formed. The linear copolymer of the carbon monoxide expressed by formula I (B denotes an ethylenically unsatd. α-olefin moiety having at least three carbon atoms polymerized via ethylenic unsaturation and the ratio of (y) to (x) denotes 0.5 to 0) is adequate.