PURPOSE: To uniformly drop a filler up to the vicinity of a side wall by making a discharging gap in the upper part of a roll feeder wider towards the vicinity of the side wall to prevent the lowering of the dropping speed of the filler in the vicinity of the side wall.

CONSTITUTION: A discharging gap (h) in the upper part of a roll feeder 6 is made wider towards the side wall of both sides in the direction of the length of the roll feeder 6. Since the lowering region of a dropping speed due to friction against the side wall in the vicinity of the side wall is generated, the lowering of the dropping speed results in the difference of the discharging quantity from the roll feeder 6. Accordingly, when the discharging gap (h) is widened in the vicinity of the side wall by the reduced quantity of the discharging quantity in the vicinity of the side wall, the lowering of the dropping speed of the filler in the vicinity of the side wall can be prevented, thereby being able to uniformly drop the filler. Namely, the larger part of friction against the side surface in the filler in the vicinity of the side wall inside a moving bed 2 as compared with the filler in the central part is adjusted by reducing resistance generated when the filler is discharged by the roll feeder 6, thereby uniforming a total dropping speed.