Provided are a method and an apparatus of controlling exhaust gas in an oxyfuel combustion boiler which can attain stable combustion of the boiler while an amount of unburned combustibles and a NOx density in exhaust gas are maintained in their allowable ranges. A rate of supply of directly supplied oxygen to burners 6 to a total amount of oxygen produced by an air separation unit 18 is regulated in an operating range where the NOx density in the exhaust gas is below a NOx limit value and the amount of unburned combustibles in the exhaust gas is blow an unburned combustible limit value to thereby attain stable combustion of the oxyfuel combustion boiler.