Provided are a copolymer having biocompatibility sufficient for use in medical material applications, and a method of forming a crosslinked body, the method including imparting biocompatibility to a substrate surface through the use of the copolymer. More specifically, a protein adsorption-suppressing effect and a cell adhesion-suppressing effect, which are features of a phosphorylcholine group, are imparted to the substrate surface. It has been found that a copolymer containing a phosphorylcholine constitutional unit and a photoreactive constitutional unit, or a copolymer containing a phosphorylcholine constitutional unit, a photoreactive constitutional unit, and a hydrophobic constitutional unit can impart a protein adsorption-suppressing effect and a cell adhesion-suppressing effect to a substrate surface through a simple approach called photoirradiation.