A copolymer comprising a monomer of formula (1) below and a monomer having a basic functional group copolymerizable with the monomer as monomer components, wherein molar ratio of the monomer of formula (1) to the monomer having a basic functional group is 85/15 to 99.9/0.1 and wherein the copolymer has a number based mean molecular weight of 5,000 to 500,000. The copolymer is excellent in antithrombotic activity and further in biocompatibility and can be used as a medial material having high hydrophilicity. It also can serve as an antithrombotic surface treating agent. (wherein R1 is an alkylene group having 1 to 4 carbon atoms, R2 is an alkyl group having 1 to 4 carbon atoms, and R3 represents hydrogen or a methyl group).