Post-processing apparatus, control method thereof and image forming system

An image forming system comprising:

• an image forming apparatus for forming an image on a sheet and for ejecting a first sheet on which an image is formed;
• and
• a post-processing apparatus which produces a booklet by applying a center-folding process or a saddle-stitching process to the first sheet ejected from the image forming apparatus,
• wherein the post-processing apparatus comprises:
• a sheet ejection section for ejecting a sheet;
• a sheet tray for storing thereon the sheet ejected from the sheet ejection section;
• a regulating section for aligning a plurality of sheets stored on the sheet tray by coming in contact with edges of the sheets;
• a drive section for moving the regulating section in a direction of sheet ejection or an opposite direction from the direction of sheet ejection;
• a control section for moving the regulating section by controlling the drive section such that a third sheet falls onto a sheet surface of a second sheet stored as a top of sheets on the sheet tray when the third sheet is ejected from the sheet ejection section in a situation where the second sheet which has a smaller size than a first sheet in the direction of sheet ejection is stored on the first sheet on the sheet tray;
• and
• a processing section for applying the center-folding process or the saddle-stitching process to a bundle of sheets including a plurality of sheets aligned by the regulating section and stored on the sheet tray.