Disclosed is a graft polymerization method which has solved problems involved in use of a solvent, a radical initiator and high-energy radiation when a monomer is graft-polymerized on the surface of a polymer base. The graft polymerization method is characterized in that a polymerization product is obtained by immersing a polymer base (i) having a ketone group on the surface into a reaction system containing a monomer (ii) and then irradiating the polymer base (i) with light so that polymerization of the monomer starts from the surface of the polymer base (i).