Abstract: The present invention is a method for suppressing corrosion of carbon steel members composing a nuclear power plant. That is, the processing solution contains a chemical including iron (II) ions, an oxidizing agent for oxidizing at least one part of the iron (II) ions into iron (III) ion, and a pH adjustment agent for adjusting pH. The pH of the processing solution is adjusted in the range of 5.5 to 9.0 by the pH adjustment agent. The processing solution is introduced into a purifying system pipe having the carbon steel members. The iron (II) ions are adsorbed on an inner surface of the purifying system pipe, namely, a surface of the carbon steel members. The ferrite film is formed on the surface of the carbon steel members by oxidizing the absorbed iron (II) ions. Therefore, corrosion of the carbon steel members is suppressed by the ferrite film.