PURPOSE: To provide a self-purifying function by covering the furnace bed with small interlinked grate pieces (about 10cm square), and the ash and sand entering in the gaps therebetween are caused to fall by relative sliding movements of the grate pieces.

CONSTITUTION: A stoker is composed of pushers 1, flexible grate 2, pusher supporting sliding beams 3, connecting rods 4, driving cylinders 5 and grate fixing pins 6. In the case where the pushers 1 travel backwards, lumps of combustibles 8 and 7 on a upper stair and a lower stair, respectively, are rotated as a whole, and their protuberant portions are broken, allowed to fall are stirred. During the transfer from the backward-traveling position to the forward-traveling position, the combustible lumps which have not fallen are broken at their central portions and mix with the precedingly falled portions. Charged from the upper side, the combustibles 10 move downwards while being stirred; on the other hand, combustion gas 9 flows upwards, heat the furnace bed surface, and is discharged out of the furnace while promoting the combustion. Combustion air 11 is fed from the lower side through the grate.