Provided are surface-modified rubbers or surface-modified thermoplastic elastomers, such as catheters, syringe barrels, tubes of medical instruments, and mudguard fenders, and methods for modifying the surface of rubber or a thermoplastic elastomer, wherein a lubricant layer is firmly bonded to the surface of medical devices such as catheters, syringe barrels, and tubes of medical instruments to impart lubricity to the surface and, further, improve the durability of the lubricant layer on the surface, thereby suppressing deterioration of the sliding properties. Included is a surface-modified rubber or a surface-modified thermoplastic elastomer each having a surface at least partially treated by polymerization of a monomer in the presence of a thermal polymerization initiator.
