The object of the present invention is achieved by providing a copolymer, in particular, a block copolymer which has excellent coating formation ability, adhesive properties to a substrate, and does not adsorb protein； an antithrombotic coating agent which has superior antithrombotic properties and adhesive properties to a substrate to those of the conventional antithrombotic coating agent； and a medical instrument which is obtained by coating the antithrombotic coating agent, and the block copolymer includes a polymer (A) containing a (meth)acrylic ester monomer and a polymer (B) containing a (meth)acrylamide monomer and has excellent coating formation ability and high adhesive properties to a substrate.
