PURPOSE: To improve a controlling performance in a steam temperature control device for a boiler by a method wherein its controlling operation is carried out under a consideration of a state of a damper in another system.

CONSTITUTION: Signals (ΔTA, ΔTB) 4a, 4b obtained by subtracting steam temperatures TA, TB from set temperatures TAS, TBS by subtractors 3a, 3b are inputted to PID controllers 5a, 5b and a control circuit 10. The control circuit 10 calculates a total control deviation R defined by control deviations ΔTA=TA-TAS, ΔTB=TB-TBS in both systems A and B, and outputs correction signals ΔYA, ΔYB to adders 17a, 17b in response to the control deviation R larger than a predetermined value RS. The adders 17a, 17b add output signals 6a, 6b of the PID controllers 5a, 5b and the correction signals ΔYA, ΔYB received from the control circuit 10 and then outputs a result of this addition as new damper opening degree instruction signals (YA, YB) 18a, 18b.