PURPOSE: To prevent entrance of cold ash charged at a late stage and wet ash to a preheat chamber by a method wherein ash fed to a furnace body is fed, in order, from ash at a bottom part in a hopper for ash charge through a charge port.

CONSTITUTION: A push-out device 16 to push out ash in a hopper 15 for ash charge in a furnace body 2 through a charge port 11 is arranged to the bottom part of the hopper 15 for ash charge. The push device 16 comprises a push-out member 17 protruded and retracted to and from the furnace body 2 through the charge port 11, and a cylinder device 18 to protrude and retract the push-out member. Thus, when the push-out member 17 of the push-out device 16 is protruded and retracted by means of a cylinder device 18, incinerated ash A at the inner bottom part of the hopper 15 for ash charge is fed, in order, to a preheat chamber 7 through the charge port 11. This constitution prevents cold ash and wet ash from entering the preheat chamber.