Patent ID: 7459085

Claim:
A hydrophilic microporous membrane comprising a thermoplastic resin, having been contacted with a hydrophilic vinyl monomer having one vinyl group after generation of radicals by irradiation with ionizing radiation in order to be subjected to hydrophilizing treatment by a graft polymerization reaction, and having a maximum pore size of 10 to 100 nm, wherein said hydrophilic microporous membrane has a coarse structure layer with a higher open pore ratio and a fine structure layer with a lower open pore ratio which are formed in one piece, wherein said coarse structure layer exists on at least one side of the membrane surface and has a thickness of 2 μm or more and a thickness of said fine structure layer is 50% or more of the whole membrane thickness, wherein when 3 wt % bovine immunoglobulin having a monomer ratio of 80 wt % or more is filtered at a constant pressure of 0.3 MPa, an average globulin permeation rate A (liter/m 2 /h) for 5 minutes from the start of filtration satisfies the following formula (1) and an average globulin permeation rate B (liter/m 2 /h) for 5 minutes from the time point of 55 minutes after the start of filtration satisfies the following formula (2): Globulin permeation rate A>0.0015 maximum pore size (nm) 2.75 (1) Globulin permeation rate B /globulin permeation rate A >0.2 (2).