Methods are provided for surface-modifying a rubber vulcanizate or a thermoplastic resin. The methods allow these objects to have a chemically fixed surface layer that exhibits not only low adsorption or selective adsorption properties with respect to proteins and cells, but also excellent durability, instead of having a coating which has drawbacks such as reduction in properties due to separation or peeling of the coating. Included is a method for surface-modifying an object made of a rubber vulcanizate or a thermoplastic resin, the method including: a step 1 of forming polymerization initiation points on the surface of the object; and a step 2 of radically polymerizing a hydrophilic monomer starting from the polymerization initiation points by irradiation with UV light having a wavelength of 300 to 400 nm in the presence of an alkali metal salt to grow polymer chains on the surface of the object.