Sheet postprocessing apparatus to which optional unit can be removably attached

A sheet postprocessing apparatus comprising:

• a sheet inlet through which a sheet on which an image has been formed by an image forming apparatus is introduced;
• a first transport lane on which the sheet introduced through the sheet inlet is transported;
• a postprocessing device that performs postprocessing on the sheet;
• a drive unit including a forward rotation motor capable of rotating forward, and configured to supply driving force for transporting the sheet,
• wherein the postprocessing device includes an optional unit mounting base,
• the optional unit mounting base is configured to allow one of a folding unit having a function to fold the sheet, and a basic unit without the folding function, to be selectively mounted,
• the drive unit includes a structure for additionally mounting a bidirectional motor capable of rotating reversely,
• when the basic unit is mounted on the optional unit mounting base, the drive unit supplies the driving force from the forward rotation motor, and
• when the folding unit is mounted on the optional unit mounting base, the drive unit supplies the driving force from the forward rotation motor and the bidirectional motor additionally mounted on the drive unit.