Provided are a method and an apparatus for controlling combustion in an oxygen fired boiler which are easily applicable to an existing air fired boiler for easy and stable control of the combustion.Oxygen with a setting amount in accordance with a boiler load demand 35 is supplied to a boiler body 4. Heat absorption amount of the boiler is measured from inlet temperature of feedwater supplied to the boiler body 4 and outlet temperature of steam. Recirculation flow rate of combustion exhaust gas 14a is controlled such that heat absorption amount 41 of the boiler body 4 becomes equal to a targeted heat absorption amount 42 to thereby control oxygen concentration in all gas guided to the boiler body 4.