A surface treating agent is comprising 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)/(B)/(C) between 70/15/15 to 98/1/1. (wherein (R 1 )s represent hydrogen or a methyl group, R 2 represents an alkyl group having 1 to 4 carbon atoms, n represents an integer of 2 to 4, and R 3 and R 4 independently represent hydrogen or an alkyl group having 1 to 3 carbon atoms respectively)
