A steam turbine plant includes a chemical supply part configured to supply a pH adjuster to feedwater to a steam generator, an adjustment part for adjusting a supply amount of the pH adjuster to the feedwater by the chemical supply part, and at least one carbon steel component that includes a pipe or a device formed from carbon steel and through which the feedwater flows, the carbon steel component being configured such that an internal temperature at least partially falls within a range of not less than 120° C. and not greater than 180° C. under load operating condition of the steam turbine plant. The adjustment part is configured to, under the load operating condition, adjust the supply amount of the pH adjuster such that pH of the feedwater in each of the at least one carbon steel component is not less than 9.8.