To reduce the cost by controlling the excess air rate in the subcritical pressure region of a partial load according to a program preset individually for each kind of fuel coal to be used thereby reducing the superheater spray capacity facility while ensuring an appropriate degree of superheat at the superheater inlet.

Coals are subjected to coal properties analysis and fixed carbon, volatile component and ash content in industrial analysis are evaluated before being classified into groups of low, intermediate and high fuel ratio coals. Each selected group is then corrected with respect to an excess air ratio curve for each fuel ratio and furnace heat absorption is controlled y switching the setting of excess air ratio curve for a load according to a program when coal of each selected group is combusted. According to the method, fluctuation in the heat absorption on the furnace wall of a boiler due to combustion of coal can be suppressed through excess air rate control at the rate of spray quantity of each load.