A sheet feeding unit to be inserted between a printer main unit and a sorting unit having a plurality of discharge trays. The sheet feeding unit includes a casing in which a sheet is housed and through which a feed path and a discharge conveyance path extend horizontally. The casing has on a face thereof adjacent to the printer main unit an exit of the feed path and an entrance of the discharge conveyance path, and on a face thereof adjacent to the sorting unit an entrance of the feed path and an exit of the discharge conveyance path. The positions of the exit and entrance of the feed path coincide with each other when seen in a conveyance direction, and the positions of the entrance and exit of the discharge conveyance path coincide with each other when seen in the conveyance direction.