The purpose of the present invention is to provide a copolymer having exceptional water permeability, platelet adherence suppression, and protein adherence suppression, and to provide a separation membrane, a medical device, and a medical separation membrane module in which said copolymer is used. The present invention provides a copolymer containing monomer units derived from two or more types of monomer, wherein the hydration energy density of the copolymer is 158.992 to 209.200 kJ.mol<；SP>；-1<；/SP>；.nm<；SP>；-3<；/SP>；, the monomer unit having the highest hydration energy density among the monomer units does not include a hydroxy group, the volume fraction of monomer units having the highest hydration energy density among the monomer units is 35 to 90%, and the difference in hydration energy density is 71.128 to 418.400 kJ.mol<；SP>；-1<；/SP>；.nm<；SP>；-3<；/SP>；.本發明之目的在於提供透水性、抑制血小板附著、抑制蛋白質附著優異之共聚物以及使用其之分離膜、醫療裝置、醫療用分離膜模組。本發明提供一種共聚物，其係含有來自2種以上的單體之單體單元的共聚物，上述共聚物之水合能密度為158.992~209.200kJ‧mol-1‧nm-3，於上述單體單元中，水合能密度最大的單體單元係不含羥基的單體單元，於上述單體單元中，水合能密度最大的單體單元之體積分率為35~90%，水合能密度之差為71.128~418.400kJ‧mol-1‧nm-3。
