Provided are: a copolymer having sufficient biocompatibility for use in medical material applications； and a method for forming a cross-linked body that uses said copolymer and imparts biocompatibility to a substrate surface. More specifically, a protein-absorption suppression effect and a cell-adhesion inhibition effect, being characteristics of phosphorylcholine groups, are imparted to the substrate surface. The present invention was completed as a result of the discovery that a copolymer containing a phosphorylcholine structural unit and a photoreactive structural unit or a copolymer containing a phosphorylcholine structural unit, a photoreactive structural unit, and a hydrophobic structural unit can impart a protein-absorption suppression effect and a cell-adhesion inhibition effect to a substrate surface by using a simple method called light irradiation.本發明提供一種用於醫療材料用途的、具有充分的生物相容性的共聚物；及使用共聚物而賦予基材表面生物相容性的交聯體形成方法。詳細而言，賦予基材表面作為磷酸膽鹼基的特長的蛋白質吸附抑制效果與細胞黏接抑制效果。本發明的發明人們發現，含有磷酸膽鹼結構單元及光反應性結構單元的共聚物、或含有磷酸膽鹼結構單元、光反應性結構單元及疏水性結構單元的共聚物，能夠藉由光照射這樣簡單的方法，對基材表面賦予蛋白質吸附抑制效果與細胞黏接抑制效果，從而完成本發明。
