A porous membrane includes a modacrylic copolymer. The modacrylic copolymer includes, with respect to 100 parts by mass of all structural units constituting the modacrylic copolymer, 15 to 85 parts by mass of a structural unit derived from acrylonitrile, 15 to 85 parts by mass of a structural unit derived from at least one halogen-containing monomer selected from the group consisting of vinyl halide and vinylidene halide, and 0 to 10 parts by mass of a structural unit derived from a vinyl monomer having an ionic substituent. The porous membrane can be produced by preparing a modacrylic copolymer solution by dissolving the modacrylic copolymer in a solvent, and bringing the modacrylic copolymer solution into contact with a non-solvent for the modacrylic copolymer such that the modacrylic copolymer solution is solidified.