To perform an aligning motion so that the ends of a plurality of sheets are nicely aligned on a tray by sorting them without displacing the positions of the loaded sheets.

If an alignment motion is carried out after alignment plates 30A and 30B are moved corresponding to the sheet discharging position on a main tray 10B, one of the alignment plates 30A and 30B is fixed and the other is reciprocated in the horizontal direction orthogonal to the discharge direction of the sheet and then both the alignment plates 30A and 30B are reciprocated in the horizontal direction orthogonal to the discharge direction of the sheet for performing the alignment motion.