Abstract: A circulating fluidized bed boiler has reduced corrosion in the exchanging tube in an external heat exchanger. The circulating fluidized bed boiler has a furnace which combusts a fuel which is fluidized together with a bed material, a cyclone dust collector into which an flue gas which is generated by the combustion in the furnace is introduced and which catches particles in the flue gas, a separation loop, in a seal box, which separates corrosive components from the particles so as not to introduce the corrosive components to the external heat exchanger which is arranged downstream of the seal box.