In an apparatus of controlling an exhaust gas in an oxyfuel combustion boiler having the boiler 4 provided with burners 6 and a two-stage combustion port 7, a primary recirculation line 12 through which pulverized coal obtained by a mill 3 is fed to the burners 6 of the boiler 4 by the primary recirculating exhaust gas, a secondary recirculation line 14 through which another portion of the exhaust gas in recirculation is fed to a wind box 5 of the boiler 4, an oxygen producer 23, a direct supply line 25 through which a portion of oxygen produced by the oxygen producer 23 is directly fed to the burner 6 and a secondary oxygen mixing line 24 through which another portion of oxygen produced by the oxygen producer 23 is fed to the secondary recirculation line 14, the apparatus comprises an oxygen supply line 26 through which oxygen is fed to the two-stage combustion port of the boiler 4 and a flow rate regulator 20, 27 in the oxygen supply line for adjusting oxygen density.