To surely align sheets by an aligning member, even when sheets stacked on a stacking tray are curled, and to prevent lowering of alignment of the sheets.

In a finisher 100, the sheets ejected by an ejection roller pair are stacked on a stacking tray 137 provided to be movable in a sheet stacking height direction so that the position of the uppermost surface of the sheets can be fixed, and the sheets stacked on the stacking tray 137 are aligned by an aligning member pair 1 moving in a width direction orthogonal to a sheet ejecting direction, and when the volume of stacked sheets stacked on the stacking tray 137 increases, the position of the aligning member pair 1 is controlled so that the sheets stacked on the stacking tray 137 can abut on the aligning member pair 1 when aligning the sheets by lowering the aligning member pair 1.