The present invention aims to provide a method for modifying a surface of a rubber vulcanizate or a thermoplastic elastomer, which can impart excellent sliding properties and excellent durability against repeated sliding motion, and allow the surface to maintain the sealing properties, without using expensive self-lubricating resins. The present invention relates to a method for modifying a surface of an object of a rubber vulcanizate or a thermoplastic elastomer, the method including: Step 1 of forming polymerization initiation points on the surface of the object; and Step 2 of radical-polymerizing a monomer starting from the polymerization initiation points to grow polymer chains on the surface of the object.