PURPOSE: To suppress an increase in unbalanced temperature at the circumferential wall face of the furnace that is created as the degree of superheating increases and the generation of drain accompanying a fall in the degree of superheating by controlling the degree of superheating (difference between fluid temperature at the outlet of furnace and the saturation temperature of the fluid) in the sub-critical pressure zone of the fluid at the outlet of a furnace in a once-through boiler the circumferential wall of the furnace of which is constituted by evaporation tubes.

CONSTITUTION: In fuel supply pipes 5 for burners 3 installed in multistages in the direction of the height of a furnace a flow rate regulating valve 4 is installed to each stage, and a bias is applied to the volumes of charged fuel in each stage.