PURPOSE:To enable stable control of a main steam temperature through correction of the change in efficiency of a final stage superheater due to a fluctuation in a main steam flow rate by a method wherein the steam temperature set value of a frontmost stage superheater temperature control means is decided according to a main steam flow rate on the the final stage superheater side. CONSTITUTION:Control is effected so that, through correction of the main steam temperature set values of temperature difference regulators 8 and 9 according to a change in a main steam temperature detected by a temperature detector 14, the change in efficiency of a final stage superheater 3 is absorbed on the first and second front stage superheaters 1 and 2 side. In a subtractor 20, flow rates F1 and F2 of cooling water to first and second stage condensers 4 and 5 are subtracted from a flow rate F0 of main steam from the final stage superheater 3. Further, in a primary function circuit 21, an output from the subtractor 20 is converted to a primary function. By deciding the primary function as the main steam temperature set value obtained by control of the temperature of the frontmost stage superheater 1, control is made so that the change in efficiency of the final stage superheater 3 due to a change in a main steam flow rate is suppressed by the frontmost stage superheater 1 side.
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