The sheet stacking-aligning apparatus or the sheet processing apparatus includes a substantially horizontal stacking tray, a rear end aligning unit for aligning a rear end of a sheet bundle on the stacking tray, and a control unit for controlling an operation of the rear end aligning unit for aligning the sheet bundle when it is in an upstream position of the stacking tray, and the stacking tray is provided substantially horizontally. It is thus made possible to increase the stacking space, thereby increasing the number of stackable sheets and to achieve the alignment of the rear end of the sheet bundle with a simple configuration, thereby improving the stacking-aligning property of the sheet bundle on the stacking tray.