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.
