An epoxy-group-containing vinyl monomer, glycidyl methacrylate, was graft-polymerized onto an electron-beam-irradiated porous sheet to form a graft chain. Graft chains can be categorized according to the formation site into the polymer brush extending from the pore surface of a porous sheet and polymer root penetrating into the polymer matrix of the porous sheet. The polymer brush and root of the porous sheet govern its protein adsorptivity and liquid permeability. The mole percentages of the polymer brush and the root were determined from the dependence of the degree of swelling on the molar conversion of epoxy groups into ion-exchange groups with water as the solvent during functionalization. This method enables us to better understand the manner in which polymer chains are grafted onto porous polymers and helps us to design porous polymers for protein purification and enzyme immobilization.