To provide a resin composition having superior rust preventiveness over a temperature range from low to high temperatures.

The resin composition contains as essential components, a synthetic resin (A), a rust preventive (B), an expanding agent (C), a carbonizing agent (D) and a flame retardant (E), and is applied to a metal member such as a steel frame to form a coating film. A synthetic resin with a weight average molecular weight of ≥50,000 which is a polymer of a monomer group including a (meth)acrylic acid alkyl ester having a ≥4C alkyl group and an aromatic monomer is contained as the synthetic resin (A).