To provide a surface modification treatment agent for forming an organic coating layer of various resins, etc., on the surface of an alloy product with excellent adhesion strength, and a surface modification method for the alloy product using the surface modification treatment agent.

The surface modification treatment agent is an aqueous solution for modifying the surface of the alloy product whose major component is iron or titanium, and comprises a chemical polishing solution, which removes an oxidation stabilization coating present on the surface of the alloy, and a modifier that couples with the active surface of the alloy product that has become active by the removal of the oxidation stabilization coating and delays the formation of the oxidation stabilization coating on the active surface. The modifier is a material for complex formation that composes a complex with a metal ion on the active surface. The material for complex formation is made up of at least one or more kinds chosen from among a gallic acid, pyrogallol, and a tannic acid.