PURPOSE: To obtain a polymer membrane useful as a medical polymer, especially an antithrombogenic material, by forming a membrane from a specified copolymer or blend, applying a suspension or solution of the above-mentioned copolymer or blend to the membrane and subsequently removing the solvent therefrom.

CONSTITUTION: A membrane is formed from a copolymer obtained by polymerizing hydrophilic and hydrophobic monomers or a blend between a polymer (e.g., polyvinyl alcohol) obtained by polymerizing a hydrophilic monomer and another polymer (e.g., polyethylene) obtained by polymerizing a hydrophobic monomer. The above-mentioned copolymer or blend is suspended or partially dissolved in a solvent (e.g., water or methyl alcohol) miscible with one or more homopolymer parts prepared by polymerization of each monomer constituting the above-mentioned copolymer or blend. The resultant dispersion or solution is applied to the above-mentioned copolymer membrane or blend membrane and the solvent is then removed therefrom, thus obtaining the objective membrane.