PROBLEM TO BE SOLVED: To provide a method for chemically decontaminating carbon steel members of a nuclear power plant by which precipitation of oxalic acid iron (II) can be suppressed and deterioration of cation exchange resin can be suppressed.SOLUTION: Both end parts of circulation pipeline of a chemical decontamination device are connected to carbon-steel piping system through which cooling water of the nuclear power plant flows (S1). Oxalic acid is injected to heated water in a surge tank provided for the circulation pipeline (S2) to create an oxalic acid aqueous solution (S4), and the oxalic acid aqueous solution is subject to bubbling of oxygen gas in the surge tank to increase dissolved oxygen concentration of the oxalic acid aqueous solution (S5). The oxalic acid aqueous solution created by injecting pH adjuster (S6) with pH2.5 at 90°C whose dissolved oxygen concentration is increased is supplied to the piping system through the circulation pipeline to perform reduction decontamination of the piping
