To perform combustion control over a boiler by PID control and to calculate an optimum manipulated variable while taking the state of pressure fluctuation into consideration.

Steam pressure SP required for the steam pressure PV of a steam collection part 12 detected by a pressure sensor 13 is supplied and on the basis of their comparison, a device number controller 14 performs PID operation; and the continuous value of a manipulated variable as the operation result is converted into the output level of boiler number control and according to the output level, an operation control part 15 controls the combustion state of a combustion part 16.