Abstract: This invention relates to a sheet post treatment apparatus having receive means for receiving sheets, align means provided with a reference member providing a reference for aligning the sheets and a moving member movable relative to the reference member to align the sheets, and control means for controlling the aligning means to cause the moving member to wait at a different position in conformity with sheet size information before the discharge of the sheets in a case where the sheets discharged from an image forming apparatus are discharged so that the centers thereof may coincide with one another irrespective of the sizes of the sheets.