The present invention provides a surface treatment agent, a coating film obtained by using the surface treatment agent, and a surface treatment method using the surface treatment agent. The surface treatment agent can form a coating film having antifouling properties (adhesion resistance to solid microparticles) for solid microparticles having both hydrophilicity and hydrophobicity, and having antifouling properties (resistance to organic vapor adhesion) for grease vapors and volatile organic compounds. A metal surface treatment agent includes an aqueous medium, aqueous medium-dispersed microparticles (A), aqueous medium-soluble resin (B), and a cross-linking agent (C). The aqueous medium-dispersed microparticles (A) are aqueous medium-dispersed microparticles including at least one combination selected from combinations consisting of acrylic resin, phenyl sulfide resin, and amide resin; the aqueous medium-soluble resin (B) is a polymer or copolymer composed of monomers or salt thereof, or a mixture containing at least one of the polymer, the copolymer, and the salt, wherein the monomer has one or more functional groups selected from a carboxyl group, a sulfonic acid group, a hydroxyl group, an amide group, and a phosphorylcholine group; and the cross-linking agent (C) has at least one aldehyde group.