To prevent a fuel amount and an air amount from being unbalanced even when the combustion amount is put back to the original combustion amount in the middle of change.

When canceling an increase in combustion amount in the middle of operation for increasing the combustion amount, a combustion amount change boiler which increases and decreases the combustion amount by increasing and decreasing the fuel amount and air amount carries on control in a direction in which the combustion amount is increased until an air supply amount reaches a set value of a decrease in fuel supply amount, and starts decreasing the fuel supply amount and decreasing the combustion air supply amount after the air supply amount reaches the set value of the decrease in fuel supply amount. When canceling a decrease in fuel amount in the middle of operation for decreasing the combustion amount, the fuel amount change boiler carries out control in a direction in which the combustion amount is decreased until the air supply amount reaches a set value of an increase in fuel supply amount, and starts increasing the fuel supply amount and increasing the combustion air supply amount after the air supply amount reaches the set value of the fuel supply amount increase.