Perforated endless conveying belts 3 are stretched between drive and idle rollers 1, 2. A suction box 8 is arranged between upper and lower belt portions 3c, 3d of the perforated endless conveying belts. Abutment members lla-lld are attached to positions of a support 10 corresponding to the perforated endless conveying belts and pressed against the perforated endless conveying belts. The upstream of the suction box from the abutment members forms an attracting area 12, and intake holes 8b are formed therein. Lifting plates 14a-14c extend to at least an intake hole range of the attracting area between the perforated endless conveying belts. The lifting plates move between a first position to vertically open the intake holes and project from conveying surfaces 3b of the perforated endless conveying belts and a second position to close the intake holes and retract from the conveying surfaces of the perforated endless conveying belts.