An object of the present invention is to obtain a protein monomer useful as a raw material for medicines industrially and economically in high yield and high purity. In the method for purifying a protein of the present invention, a protein solution containing a protein monomer and a protein aggregate is passed through a column holding a porous rigid polymeric self-supporting structure to which a hydrophobic group is immobilized, and then recovering the protein monomer in a flow-through mode.