Provided is a method for surface-modifying a rubber vulcanizate or a thermoplastic elastomer, which makes it possible to impart excellent sliding properties, durability after repeated sliding, and properties to prevent adsorption or aggregation of proteins, and further maintain good sealing properties for a long time, without using expensive self-lubricating resins. Also provided are surface-modified elastic bodies, including medical devices, e.g. a gasket for syringes, catheter, or blood or body fluid analyzer, etc., at least part of whose surface is modified by the method. Included is a method for surface-modifying an object made of a rubber vulcanizate or a thermoplastic elastomer, the method including: step 1 of forming polymerization initiation points on a surface of the object； step 2 of radically polymerizing a monomer starting from the polymerization initiation points to grow polymer chains on the surface； and step 3 of irradiating the grown polymer chains with an electron beam.
