Post-processing device and image forming system

An image forming system comprising:

• an image forming apparatus forming an image on a sheet;
• and
• a post-processing device executing post-processing to the sheet having the image formed by the image forming apparatus,
• wherein the image forming apparatus includes:
• a scanner obtaining image data by reading the sheet;
• an image forming part forming the image on the sheet;
• and
• a sheet ejecting part ejecting the sheet having the image formed by the image forming part,
• the post-processing device includes:
• a post-processing conveying path introducing the sheet ejected from the sheet ejecting part and conveying the sheet;
• and
• a stapling part collecting sheets conveyed on the post-processing conveying path for each predetermined number of sheets and executing staple processing to the sheets,
• the image forming apparatus or the post-processing device further includes a size deciding part deciding whether the sheet is a regular size or an irregular size, and, if the sheet is the irregular size, deciding a sheet size of the sheet on the basis of the image data obtained by the scanner,
• the post-processing device further includes a staple controlling part determining, if the sheet is the regular size, a binding position of the sheet directed to the staple processing by a previously determined binding position, and, on the other hand, determining, if the sheet is the irregular size, the binding position of the sheet directed to the staple processing on the basis of the sheet size decided by the size deciding part.