To contrive the reduction of the increase of a metal temperature as well as a difference (maximum value-average temperature) due to unbalance of furnace heat absorption in a variable pressure once-through boiler.

In a variable pressure once-through boiler equipped with superheater temperature reducers 6, 8, the rise 25 of the maximum value of a furnace water-cooled wall metal temperature and/or the increase 24 of a temperature difference between the maximum value and an average value are detected to regulate the flow rate of passing fluid of the superheater temperature reducer 6 by the detecting output, whereby the flow rate of passing fluid of the furnace water-cooled wall is changed to reduce the maximum value of the furnace water-cooled wall metal temperature and/or a temperature difference between the maximum value and the average value. Further, in a variable pressure once-through boiler equipped with a gas re-circulation system, the rise of the maximum value of the furnace water-cooled wall metal temperature and/or the increase of a temperature difference between the maximum value and the average value are detected to regulate the flow rate of re-circulation of gas at the regulator 11 and change the heat absorbing amount of the furnace water-cooled wall, whereby the maximum value of the furnace water-cooled wall metal temperature or a temperature difference between the maximum value and the average value is reduced.