There are provided a plurality of sheet feeding units 2a to 2d arranged in a vertical direction at spacing therebetween, a vertical sheet conveyance path G arranged oppositely with the sheet outlets 12a to 12d of the sheet feeding units 2a to 2d, conveyance means 1 and 6 for sequentially conveying the sheets fed from respective sheet feeding units 2a to 2d, vertically downward along the sheet conveyance path G, and control means controlling the sheet feeding units 2a to 2d and the conveyance means 1 and 6 in such a manner that the sheets fed from the sheet feeding units 2a to 2d are conveyed while at least partially overlapping the sheets which are fed from a sheet feeding unit of a next lower site on the sheets which are fed from the sheet feeding unit of an upper site. Imaging means 3a to 3d are arranged oppositely with the sheets conveyed on the sheet conveyance path G, and the control means controls the motions of the imaging means 3a to 3d and detects collation errors based on image data captured from the imaging means 3a to 3d.