PURPOSE: To reduce the supply of gas from a hot air generating furnace by recirculating a part of an exhaust gas through a furnace to achieve the recovery of heat from the exhaust gas when a fluidized bed boiler is started.

CONSTITUTION: When starting a boiler, a combustion gas is blown in from an air feed tube 5 and a hot air generating furnace through a fan 14 and a damper 15 and a medium on a dispersion plate 2 is heated and moved down where the exhaust gas thereof is discharged out of the furnace from a tube 10. Then, a combustion assistant oil is fed into the furnace and burned and when the temperature in the furnace is raised properly, coal particles are fed thereinto from a fuel feed tube 6 and ignited to burn. In this process, the exhaust gas is recirculated to the air feed tube 5 with a fan 12 passing through a branch tube 11 from the tube 10. Here, the quantity of recirculation gas is adjusted with a damper 15 according to the temperature in the furnace. On the other hand, the supply of hot air from the air feed tube 5 is regulated with a damper 15 according to the quantity of the exhaust gas to be recirculated.