To enable accurate and stable control of the temperature of reheated steam at an outlet of a reheater by determining an RH gas damper opening command by adding an RH gas damper corrected opening to an RH gas damper basic opening which is determined from a furnace heat recovery rate and a power generation output command by partially-linear nonlinear conversion.

In a computer 27, an RH gas damper basic opening is determined on the basis of a weighting factor and a basic function and of a furnace heat recovery rate and a power generation output command which are given from an arithmetic unit 19. The RH gas damper basic opening thus determined is linear partially and nonlinear as a whole in relation to the furnace heat recovery rate. In an adder 26, the RH gas damper basic opening from the computer 27 and an RH gas damper corrected opening from a proportional integration regulator 25 are added up and thereby an RH gas damper opening command is determined. The determined RH gas damper opening command is given to a driving device 9 and the opening of a reheater gas damper 8 is regulated.