Abstract: A method is disclosed for purifying an antibody monomer, comprising providing a porous membrane comprising a hydrophobic porous substrate, a hydrophilic molecular chain of a different material from that of the porous substrate, immobilized on the surface of pores of the porous substrate, and a side chain of the molecular chain, containing a nitrogen atom to which one to three alkyl groups each having two or three carbon atoms are bonded; passing an antibody solution containing antibody aggregates of dimers or higher-order multimers through the porous membrane to adsorb the antibody aggregates to the porous membrane; and recovering the purified antibody monomer in the passed solution.