The invention relates to a method for retrofitting a pulverized coal boiler in an air combustion boiler system having a plurality of burners (7a, 7b, 7c) over a plurality of stages, wherein an air separation unit (17) for separating oxygen from air and a CO 2 recovery device (16) is installed. It is proposed that the method comprises the steps of: stopping to supply fuel into the furnace (1) from a burner located in a middle stage among the plurality of stages; installing a new burner to an opening of the after-air port (9) of the pulverized coal boiler; and installing a supply line to introduce the oxygen manufactured by the air separation unit (17) and part of the recirculation gas to the plurality of stages of burners (7a, 7b, 7c) except for the stopped burner and to the new burner through the supply line, and supplying the oxygen and the recirculation gas into the furnace (1).