The invention relates to a metal surface treatment agent used for forming a surface treatment coating. Even if a resin film is laminated onto a substrate surface made from stainless steel, and molding is subsequently carried out, the metal surface treatment agent imparts a high adhesion to the laminate film, so that the laminated film is prevented from peeling and can maintain a stable adhesion over the long term, regardless of exposure to high-boiling gas atmosphere, solvent and acid. The metal surface treatment agent contains a Cr(III) compound (A), and at least one type of compound (B) selected from an inorganic compound (b1), a film-forming inorganic compound (b2) and an organic chelate compound (b3) which can bond to Cr (III) compound (A) to form a film. The organic compound (b1) has film-forming properties and is at least one from an anion compound and a nonionic compound. If the metal Cr equivalent mass of compound Cr (III) compound (A) is set to M, and the mass of compound (B) is set to N, the ratio N/M is set to fall within the range of 0.005 to 1.