A sheet processor includes a processing tray with a first surface that receives sheets and forms a pile of sheets. An endless belt circulates by way of first and second surface sides on a side opposite the first surface of the processing tray. A first roller reverses a circulation path of the endless belt from the first surface side to the second surface side. A second roller is disposed at an interval from the first roller and reverses the circulation path from the second surface side. First and second abutments are attached to the belt and abut an end edge of the pile of sheets. The abutments are attached to the belt so that the first abutment is positioned near the first roller and on the second surface side and the second abutment is near the second roller on the second surface side.