The present invention provides methods of modifying the surfaces of a plurality of three-dimensional objects at the same time to provide good sliding properties, good durability after repeated sliding, good sealing properties, and the like, and also provides gaskets for syringes obtained by such methods. The present invention relates to a method for modifying the surfaces of a plurality of three-dimensional objects including: a step 1 of immersing a plurality of three-dimensional objects in a photopolymerizable monomer-containing liquid； and a step 2 of polymerizing the photopolymerizable monomer by photoirradiation while rotating a vessel containing the plurality of three-dimensional objects and the photopolymerizable monomer-containing liquid, to grow polymer chains on the surfaces of the plurality of three-dimensional objects.
