A circulation pipe 29 of a chemical decontamination apparatus 28 including a malonic acid injection apparatus 32 and an oxalic acid injection apparatus 37 is connected to a purification system pipe 21, which is made of carbon steel, of a boiling water nuclear power plant. A malonic acid aqueous solution is injected from the malonic acid injection apparatus 32 into the circulation pipe 29. An oxalic acid aqueous solution is injected from the oxalic acid injection apparatus 37 into the circulation pipe 29. A reduction decontaminating solution including a malonic acid of 5200 ppm and an oxalic acid within a range of 50 to 400 ppm is supplied into the purification system pipe 21 through the circulation pipe 29. Reduction decontamination for an inner surface of the purification system pipe 21 is executed. After the reduction decontamination for the purification system pipe 21 finishes, the malonic acid and oxalic acid included in the solution are decomposed and furthermore, the solution is purified.