A surface treating agent includes a copolymer including: a monomer (A) having a hydrophilic functional group of formula (1) or (2) below, a monomer (B) having a basic functional group, and a monomer (C) having a reactive functional group as monomer components, wherein the copolymer has a molar ratio of the monomer (A)/the monomer (B)/and the monomer (C) being between 70/15/15 and 98/1/1.(wherein each of R1 represents a hydrogen atom or a methyl group, R2 represents an alkyl group having 1 to 4 carbon atoms, n represents an integer of 2 to 4, and each of R3 and R4 independently represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms).
