An agent for providing an electrostatic coating property and improving the water resistance of a coating film comprises 100 weight parts of an aromatic ring-containing polyetheresteramide (A) derived from a polyamide having carboxyl groups at both ends and having a number average molecular weight of 500 to 5,000 and an alkylene oxide adduct of bisphenols having a number average molecular weight of 1,600 to 3,000; and 5 to 100 weight parts of a vinyl copolymer comprising as essential constituent units a vinyl monomer (b1) having a sulfonic acid (or sulfonate) group and at least one monomer (b2) selected from vinyl monomers having a functional group that is reactive with (A). Thus, an agent for providing an electrostatic coating property and improving the water resistance of a coating film that allows electrostatic coating immediately after molding without the coating of a conductive primer for under coating in electrostatically coating a styrene-based resin molded article and provides an excellent water resistance for a coating film after coating, and an electrostatically coated resin molded article using the same are provided.