PROBLEM TO BE SOLVED: To provide a polymeric surface treatment agent for conferring fouling resistance onto the surface of a metal, ceramic (including metal oxide), etc. and a surface treatment method using the surface treatment agent.SOLUTION: The provided copolymer is a copolymer including a monomer unit possessing, as a side chain thereof, a phosphorylcholine group and a monomer unit possessing, as a side chain thereof, an amino group wherein a dihydroxyphenyl group is included as an intramolecular member. It is desirable for the dihydroxyphenyl group to exist at the terminal of the copolymer and to comprise a catechol group.
