A surface treatment agent including a copolymer having a unit (A) based on a polymerizable vinyl monomer having a reactive silyl group and a unit (B) based on a (meth)acrylamide compound having a biocompatible functional group, wherein a ratio of the unit (A) with respect to all units constituting the copolymer is from 0.05 to 25 mol %, and a ratio of the unit (B) with respect to all units constituting the copolymer is from 30 to 99.95 mol %.