The purpose of the present invention is to provide: a copolymer having blood compatibility and antithrombotic properties of greatly suppressing protein adhesion so as to be usable even when being in contact with a biological component such as blood for a long period of time； and a medical device using the copolymer. In order to achieve the purpose, the present invention has the following configuration. Specifically, the copolymer is characterized by including a hydrophilic unit and a hydrophobic unit, wherein the hydrophobic unit contains at least one type of a carboxylic acid vinyl unit, and the number of carbon atoms at the terminal of a side chain of the carboxylic acid vinyl unit is 2-7.本發明之目的在於提供一種即使長時間與血液等生物成分接觸亦可使用之具血液相容性與大幅抑制蛋白質附著之抗血栓性的共聚物，以及使用其之醫療裝置。為了達成前述目的，本發明具有以下之構成。即一種共聚物，其係包含親水性單元與疏水性單元，特徵在於前述疏水性單元至少含有一種類之羧酸乙烯酯單元，前述羧酸乙烯酯單元之側鏈末端碳數為2以上7以下。
