PROBLEM TO BE SOLVED: To provide a method for modifying a pulverized coal boiler of an oxygen combustion boiler system reducing a heat load to a furnace wall of the pulverized coal burner, and capable of inexpensively maintaining structural reliability by a short construction period. SOLUTION: In the method for modifying the pulverized coal boiler into the oxygen combustion boiler system by installing an oxygen manufacturing device, and a CO 2 recovery device recovering, separating CO 2 gas from combustion exhaust gas discharged from the boiler, a burner of a middle stage of plural stages of burners installed in a furnace of the pulverized coal boiler is halted, a new burner is installed in an opening for after air port installment of a furnace wall surface, one part of recirculation gas circulating oxygen manufactured by the oxygen manufacturing device and one part of exhaust gas discharged from the pulverized coal boiler to the pulverized coal boiler is conducted to the plural stages of burners excluding the halted burner and the new burner provided in the opening for after air port installment to supply the oxygen and the recirculation gas into the furnace and to adjust a temperature in the furnace. COPYRIGHT: (C)2011,JPO&INPIT