The present invention addresses the problem of providing a porous separation membrane that, even after a long-term use, shows little decrease in protein permeability.　The porous separation membrane is obtained: by having an asymmetrical structure in which one surface of the membrane is a dense layer and the other surface is a coarse layer; and by having a biocompatible polymer supported in the membrane. In a TOF-SIMS surface analysis on a cross-section that includes the dense layer and the coarse layer, (1) and (2) are satisfied. (1) The smallest value of the standardized intensity of ion signals derived from the biocompatible polymer in the coarse layer is at least 0.15 times the greatest value. (2) The standardized average intensity of ion signals derived from the biocompatible polymer in the dense layer is at least 2.0 times the standardized average intensity of carboxylate ion signals of the coarse layer.