There provided is a post-processing apparatus which has a simple structure and makes more than one sheet of recording paper easily slide down a slope of a post-processing tray, has no irregularity in the alignment of the sheets of recording paper in the discharging direction, and makes the alignment of sheets of recording paper easier after that. A finisher unit is provided with conveyance rollers for discharging sheets to a processing tray, aligning plates for guiding both sides of the sheets discharged by the conveyance rollers, and a control CPU for controlling an operation of the aligning plates. When the sheets guided by the pair of aligning plates slide down a slope of the processing tray, the control CPU causes the pair of aligning plates to vibrate in the cross direction of the sheet and in the same direction.