To provide a rustproof structure for a pregrouting steel material, capable of surely putting curing timing to control, free of leakage of the rustproofing agent used, with low toxicity of the rustproofing agent and low environmental burden while ensuring a structural design owing to making a sure curing after undergoing tension, and to provide such a rustproofing agent for the pregrouting steel material.

The rustproof structure comprises a pregrouting material constructed by inserting wires into a sheath and filling a rustproofing agent in the spaces between the sheath and the wires; wherein the rustproofing agent includes a radical-curable one-part resin composition including: a monofunctional and/or polyfunctional (meth)acrylic monomer and a peroxide. In the structure, a material for the sheath consists favorably of polyethylene, polyvinyl chloride, FRP or polypropylene.