To provide a coating agent capable of forming a coating layer of good adhesion to a polyester film such as polyethylene terephthalate, without treatment such as primer coating to give adhesion to the film surface.

This resin composition for coating the polyester film includes a polymer (A) produced by radical polymerization of vinyl monomers which include an aromatic group-containing vinyl monomer (for example styrene) and an α,β-unsaturated dicarboxylic acid derivative (especially a fumarate derivative) as essential components.