In a boiler having plurality of burners arranged on a furnace wall of a furnace, each burner includes a cylindrical fuel nozzle for injecting a mixture of fuel and carrier gas therefor into the furnace; one or more cylindrical air nozzles provided on the outer circumference of the nozzle for injecting combustion air into the furnace, and a wind box for supplying combustion air to the nozzles in common. The wind box is provided with openings through which combustion air flows in from one direction perpendicular to the axial direction of the burner, and is partitioned by a partition wall to form plurality of parallel flow paths for the air flowing in through the openings. Some of the plurality of flow paths are connected to an upper part of the combustion air nozzle, and the other flow paths are connected to a lower part of the nozzle.