A method for rust-proofing a mold, which is capable of reducing the time and the cost required for rust-proofing the mold. The method includes an iron hydroxide-forming step for forming iron hydroxide on a predetermined part of the surface of the mold, and a surface-treating step for forming a film covering the molding surface of the mold, and for changing the iron hydroxide formed on the mold into black rust, by heating the mold, under an oxygen-deficiency atmosphere, on which the iron hydroxide is formed in the iron hydroxide-forming step.