Provided are a main steam piping connecting a steam generator and a steam turbine, a bypass piping branched from the main steam piping and bypassing the steam turbine, a bypass valve provided in the bypass piping, a warming piping branched from the bypass valve, a warming valve provided in the warming piping, and a control system. The control system controls the warming valve in such a manner that bypass valve temperature t is brought to within a temperature range satisfying the three conditions: (1) being equal to or higher than the saturated temperature of steam flowing into the bypass valve; (2) having a temperature difference from the flowing-in steam of equal to or less than an allowable value; and (3) being equal to or lower than a temperature at which the formation rate of steam oxidation scale rises.