To provide an oxygen combustion system and an oxygen combustion method, which reduce NOx in a combustion exhaust gas, and which reduce the corrosion of a recirculation duct, while adjusting an exhaust gas flow rate supplied to a CO₂ recovery device so as not to exceed a capacity of the CO₂ recovery device, even if the CO₂ recovery device has the limited capacity.

The oxygen combustion system generates steam by combusting fuel by an oxygen combustion boiler 1, and also comprises: a flue 30 located between a desulfurization device 6 and the CO₂ recovery device 8; a bypass flue 28 connected to the flue 30 at the upstream side of the CO₂ recovery device 8 after being branched from the flue 30; a control damper 29 arranged at an inlet of the bypass flue 28 and controlling a bypass exhaust gas rate; a condenser 7 arranged at the bypass flue 28 and having a refrigerant supply amount adjusting valve 26; and a control system for adjusting either of the adjusting valve 26 and the damper 29 on the basis of a temperature and a flow rate of the exhaust gas measured by a measurer 20 which is arranged at the flue 30 at the front flow side of the CO₂ recovery device 8 being the outlet side of the bypass flue 28.