An object of the present invention is to provide a porous membrane which can recover useful components while suppressing clogging during filtration of a protein solution and which has less eluate from the membrane even when the aqueous solution is filtered . The present invention is a porous membrane containing a hydrophobic polymer and a water-insoluble hydrophilic polymer, wherein the porous membrane has a dense layer at the downstream portion of filtration of the membrane, and the average pore diameter of the pores is filtered from the downstream portion of the filtration Wherein the porous film has an inclined asymmetric structure which increases toward the upstream portion and the inclination index of the average pore diameter from the dense layer to the coarse layer is from 0.5 to 12.0.本発明の目的は、タンパク質溶液濾過中の目詰まりを抑制しつつ、有用成分を回収することができ、及び水溶液を濾過しても膜からの溶出物が少ない多孔質膜を提供することである。 本発明は、疎水性高分子と非水溶性の親水性高分子を含有する多孔質膜であって、膜の濾過下流部位に緻密層を有し、細孔の平均孔径が濾過下流部位から濾過上流部位に向かって大きくなる傾斜型非対称構造を有し、及び緻密層から粗大層への平均孔径の傾斜指数が0.5~12.0である、多孔質膜を提供する。
