A sheet treating apparatus including a sheet discharging device for discharging a sheet, a first stacking tray for receiving the sheets discharged by the sheet discharging device, an aligning device for aligning a sheet bundle on the first stacking tray by a pinching movement effected by a first and second aligning members shiftable independently in a direction perpendicular to a sheet discharging direction, and a transferring device for transferring the sheet bundle on the first stacking tray to a second stacking tray, wherein, in the first stacking tray, alignment positions of the respective sheet bundles are offset by shifting the alignment positions by a predetermined amount to first and second aligning positions alternately by the first and second aligning members, and wherein the first and second aligning positions of the first stacking tray are opposite directions transverse to the sheet discharging direction with respect to the position of the sheet discharged.