To prevent significant fluctuation in the temperature of a main steam when altering the opening of a reheater gas damper by a precedence command for the reheater gas damper during the increasing or decreasing of the output command for a dynamometer.

This main steam temperature control method controls a spray flow rate to be supplied into a main steam based on a spray flow rate command 35 obtained from an output command 27 for a dynamometer. The spray flow rate command 35 is corrected by a spray flow rate precedence correction signal 55 obtained based on a reheater gas damper precedence command 54 to increase the spray flow rate during the decreasing of the opening of a reheater gas damper or decrease it during the increasing of the opening of the reheater gas damper.