The purpose of the present invention is to provide: a copolymer that is capable of being immobilized on a base material while maintaining a high antithrombotic property persistently without having to introduce heparin or an anticoagulant drug to the surface thereof; and a medical material using same. Provided is a copolymer that comprises monomer unit A, monomer unit B, and monomer unit C represented in in chemical formula 1, wherein monomer unit C has a mole fraction of 0.5-40% with respect to the total amount of all constituent monomer units.