Provided is a surface treatment agent for a metal material with which it is possible to produce a surface-treated metal material that has excellent initial anti-rust properties without being coated with an anti-rust oil, excellent lubrication properties, and excellent anti-corrosion and adhesion properties in a coating film for which phosphate-based chemical conversion treatment, zirconium-based chemical conversion treatment, or other coating underlay processing is omitted. This surface treatment agent for a metal material contains: a zirconium compound; an aqueous epoxy resin having in the skeleton thereof a carboxyl-group-containing polymer that includes a repeated unit derived from a carboxyl-group-containing vinyl monomer, said aqueous epoxy resin also having an acid value of 5-50 mg KOH/g; and a hydroxycarboxylic acid.