Block copolymer and antithrombotic coating agent

A block copolymer including a polymer (A) containing a monomer (a) represented by the general formula (1) and a polymer (B) containing at least one monomer (b) selected from the group consisting of monomers represented by the following formulae (2) to (7):

• In the formulae (1) to (7), R0 represents an alkyl group having 1 to 3 carbon atoms;
• R1 represents a hydrogen atom, or a methyl group;
• R2 and R7 independently represent an alkylene group having 2 or 3 carbon atoms;
• R3, R4, R5, and R6 independently represent a hydrogen atom, or an alkyl group having 1 or 2 carbon atoms;
• X represents a monovalent anion selected from the group consisting of —CO2−, —SO3−, —OSO3−, —OSO2−, —OP(═O)(OR8)O−, —OP(═O)(R8)O−, —P(═O)(OR8)O−, and —P(═O)(R8)O−;
• R8 represents an alkyl group having 1 to 3 carbon atoms;
• and n represents an integer from 1 to 9.)