A bundle claw attached to a sheet conveyor belt includes a concave portion having a bottom surface for regulating a position of a trailing end of the sheets, and, in the bottom surface, a surface on a lower side that comes into contact with the sheet conveyor belt is formed deeper than a surface on an upper side to form the step portion between the surface on the lower side and the surface on the upper side. Therefore, when a small number of sheets are discharged from a processing tray to a stacking tray, a trailing end of the sheets is located on the lower surface side of the concave surface of the bundle claw, where the trailing end of the sheets should be originally located, and does not move to the upper surface side of the concave surface of the bundle claw.