The purpose of the present invention is to provide a porous membrane that has both high water permeability and excellent protein fractionation performance. Provided is a method for producing a porous membrane, said method comprising a step for discharging a membrane-forming dope that contains a hydrophilic polymer from a slit formed in a mouthpiece, and a step for, after the passage of the discharged membrane-forming dope through a dry part, solidifying the membrane-forming dope in a coagulation bath to give a porous membrane, wherein the cross-section area of the slit is 3-30 times inclusive as large as the cross-section area of the solidified porous membrane.本發明之目的在於提供一種兼具透水性能與蛋白質區分性能之多孔質膜。本發明提供一種多孔質膜之製造方法，其係包括自形成於噴嘴之狹縫吐出含有親水性高分子之製膜原液之步驟、及使所吐出之上述製膜原液通過乾式部後於凝固浴中固化為多孔質膜之步驟者，並且上述狹縫之截面面積為經固化之上述多孔質膜之截面面積之3倍以上且30倍以下。
