The object is to suppress a decrease of thickness due to corrosion of structural members and to achieve a removal of radionuclides with good efficiency in a nuclear power plant. Oxidation decontamination is first conducted. An aqueous potassium permanganate solution is supplied from a circulation line 3 to a reactor pressure vessel 36, which is a stainless steel structural member, and a reactor water cleanup system piping 40 and a drain piping 46, which are carbon steel structural members. These structural members are oxidation-decontaminated by the action of potassium permanganate. Then the above-mentioned structural members are reduction-decontaminated by using an aqueous oxalic acid solution. The aqueous oxalic acid solution contains hydrazine.
