In order to achieve stable operation of an oxygen combustion boiler and to achieve an increase in the margin for a forced pressure fan or a reduction in the size of the forced pressure fan, a control is performed whereby, when a boiler load command changes so as to reach a target value A1 from a starting value A in a target time T1, the amount of oxygen supplied from an oxygen manufacturing device is adjusted such that the inlet oxygen concentration at the boiler main body inlet reaches the arrival point B' of the inlet oxygen concentration target value B1 within the inlet oxygen concentration adjustment range (23) from a standard value B in the target time T1, and after the oxygen concentration reaches the arrival point B', a control is performed whereby the inlet oxygen concentration is returned to the standard value B at a return point B'', with the control being at a change rate C1 that is smaller than the change rate C1' for the case wherein the starting value F for the air volume of the forced pressure fan reaches a target value F1 in the target time T1.