Abstract: There is disclosed a composite membrane comprising [A] a polysulfone type porous membrane capable of rejecting 90% or more of dextran molecules having a weight average molecular weight of 70,000 and [B] a specific styrene type resin coated on at least one side of the polysulfone type porous membrane in a thickness of 5 .mu.m or less and in an amount of 1.times.10.sup.-7 g/cm.sup.2 of the polysulfone type porous membrane, the styrene type resin being selected from the group consisting of (I) a specific styrene type linear polymer having a weight average molecular weight of 1.times.10.sup.-5 or more, (II) a specific blend polymer prepared by blending at least two different styrene type linear polymers each as defined above and (III) a specific crosslinked polymer selected from the group consisting of (i) a crosslinked polymer prepared by crosslinking a styrene type linear polymer having a weight average molecular weight of 1.times.10.sup.