The present invention relates to a resin surface treating agent containing as an effective component at least one cerium compound selected from tetravalent and trivalent cerium compounds. The surface treating method of the present invention is a method of bringing a surface of a resin into contact with a surface treating agent containing as an effective component at least one cerium compound selected from tetravalent and trivalent cerium compounds, and thereafter treating the surface with an acidic aqueous solution. This activates the resin surface and thereby increases a strength of adhesion of, for instance, a polyimide-based resin film with metal wires, as well as a strength of adhesion of a polyimide-based resin film with another resin. Thus, a resin surface treating agent that provides excellent productivity and reduces processing costs, as well as a surface treating method using the same, are provided.