To provide a post-processing apparatus for preventing a sheet from becoming misaligned in dropping the rear end side of the sheet carried in a processing tray from a sheet discharge path to store on the tray, and enabling the mechanism to be simplified, compact and configured at low cost, a sheet guide that guides a sheet from the sheet discharge path to the processing tray is comprised of a pair of right and left guide members, at the same time in the processing tray are disposed a pair of right and left side edge alignment members, and each guide member and each side edge alignment member are configured to shift to positions in the sheet width direction in an integral manner using a common drive motor.