PURPOSE: To uniformly cool ash and to simplify a structure of an ash discharging system by providing a fluidized bed type ash cooler for containing ash in collected exhaust gas.

CONSTITUTION: Ash in exhaust gas collected by a cyclone 4 and a high temperature dust removing unit is supplied directly to a fluidized bed type two-stage ash cooler 20. The cooler 20 has air chambers 23a, 23b having many nozzles for fluidizing the ash stepwisely in a lower part to receive the ash while fluidizing the ash by the chamber 23a of an inlet side to uniformly disperse to cool it. The ash is further fluidized in the chamber 23b of an outlet side and cooled. Thus, a heat transfer efficiency is improved to uniformly cool the ash, and a structure of the ash discharge system can be simplified.