A boiler comprising: a furnace for burning solid fuel and air; a heat exchanger (551,552;553,554;555,556,557) for collecting heat by a heat exchange inside the furnace; a fuel nozzle that is arranged to blow a fuel gas obtained by mixing solid fuel with primary air into the furnace, wherein the fuel nozzle has plural flame stabilizing members disposed as a flame stabilizer inside the fuel nozzle to realize an inner flame stabilization of the combustion flame; a secondary air nozzle that is arranged to blow secondary air from the outside of the fuel nozzle to the furnace; an additional air nozzle that is arranged to blow additional air to the upper side of the fuel nozzle and the secondary air nozzle in the furnace; a tertiary air nozzle that is arranged to blow tertiary air from the outside of the secondary air nozzle to the furnace; an air amount adjusting device (541,542,543,544,545,546,547,568) that is arranged to adjust the amount of the air supplied to the fuel nozzle, the secondary air nozzle, the tertiary air nozzle, and the additional air nozzle; and a control device that is arranged to control the air amount adjusting device (541,542,543,544,545,546,547,568) in response to a volatile content of the solid fuel so as to adjust a distribution of the total air amount of the primary air and the secondary air, and a total air amount of the tertiary air and the additional air, wherein the fuel nozzle, the secondary air nozzle and the tertiary air nozzle have a structure to supply the fuel gas, the secondary air and the tertiary air as a straight flow to form the combustion flame, wherein the flame stabilizer is formed in a double-cross split structure in which two flame stabilizing members following the horizontal direction and two flame stabilizing members following the vertical direction are disposed in a cross shape to divide the fuel gas passage into nine segments, wherein widened portions are formed in front end portions of the respective flame stabilizing members downstream in the fuel gas flowing direction, and wherein front end surfaces of the widened portions of the front end portions of the flame stabilizing members are disposed so as to be flush with an opening portion of the fuel nozzle. a furnace for burning solid fuel and air; a heat exchanger (551,552;553,554;555,556,557) for collecting heat by a heat exchange inside the furnace; a fuel nozzle that is arranged to blow a fuel gas obtained by mixing solid fuel with primary air into the furnace, wherein the fuel nozzle has plural flame stabilizing members disposed as a flame stabilizer inside the fuel nozzle to realize an inner flame stabilization of the combustion flame; a secondary air nozzle that is arranged to blow secondary air from the outside of the fuel nozzle to the furnace; an additional air nozzle that is arranged to blow additional air to the upper side of the fuel nozzle and the secondary air nozzle in the furnace; a tertiary air nozzle that is arranged to blow tertiary air from the outside of the secondary air nozzle to the furnace; an air amount adjusting device (541,542,543,544,545,546,547,568) that is arranged to adjust the amount of the air supplied to the fuel nozzle, the secondary air nozzle, the tertiary air nozzle, and the additional air nozzle; and a control device that is arranged to control the air amount adjusting device (541,542,543,544,545,546,547,568) in response to a volatile content of the solid fuel so as to adjust a distribution of the total air amount of the primary air and the secondary air, and a total air amount of the tertiary air and the additional air, wherein the fuel nozzle, the secondary air nozzle and the tertiary air nozzle have a structure to supply the fuel gas, the secondary air and the tertiary air as a straight flow to form the combustion flame, wherein the flame stabilizer is formed in a double-cross split structure in which two flame stabilizing members following the horizontal direction and two flame stabilizing members following the vertical direction are disposed in a cross shape to divide the fuel gas passage into nine segments, wherein widened portions are formed in front end portions of the respective flame stabilizing members downstream in the fuel gas flowing direction, and wherein front end surfaces of the widened portions of the front end portions of the flame stabilizing members are disposed so as to be flush with an opening portion of the fuel nozzle.