PURPOSE: To permit effective control of a fluctuation in a steam temperature, by a method wherein a change in a process amount related to a fluctuation in a steam temperature in a heating pipe group detected for each constituting element, and the detecting value is controlled to within a specified value.

CONSTITUTION: When applying to a second group 13 of a superheater group, alarming setters 23, 28, and 31 detect as the process condition of a plant whether each detecting result deviates from a specified value by the use of a furnace draft detector 102a, a main steam temperature detector 102b, and a main steam pressure detector 102c. A detected process state amount around a superheater is converted into a current by the use of a superheater inlet temperature detector 102f. A difference between a signal therefrom and a signal from a converter 25 producing a temperature signal at the outlet of the superheater is deducted by a deductor 33, it is detected by a logic 24 whether an alarm setter 34 deviates from a set value, and on the condition that the deviation does not occur, a the soot lower of a second group around the superheater is operated. During disordered control of a plant, turbulence due to blowing of a soot blower is prevented from occurring and careful operation of the soot blower is effected at each group, and this reduces a fluctuation in a steam temperature during operation of the soot blower and improves controllability of a plant.