This invention relates to a nuclear fuel reprocessing plant for processing a spent nuclear fuel with a nitric acid solution, which is provided with an electrolytic reduction apparatus for reducing the valence of oxidizing metal ions consisting of the nuclear fuel dissolved in the nitric acid solution while their ionic states are kept. The invention relates also to a method of preventing corrosion of a nuclear fuel reprocessing plant for processing a spent nuclear fuel with a nitric acid solution, wherein the valence of oxidizing metal ions consisting of the nuclear fuel dissolved in the nitric acid solution is reduced while their ionic states are kept and electrolytic reduction is carried out so as to generate NOx gas. Thus, the objects of the invention are to provide a nuclear fuel reprocessing plant having a corrosion-proofing system which prevents the corrosion of the apparatus without permitting the precipitation of corrosive metal ions in the nitric acid solution in the form of metal, and the method of corrosion-proofing such a plant.