To efficiently fold a plurality of sheets with high quality.

The image forming system includes: a folding mechanism that forms a middle folding line on a sheet discharged from an image forming device; a normal conveyance mechanism for conveying the sheet with the middle folding line to a post-processing part; a reversal conveyance mechanism for turning over the sheet with the middle folding line to the folding mechanism again; an image formation control part for setting an image formation order for each page according to a predetermined number of times that the sheets to be used for image formation are folded in the middle when images for a plurality of pages are formed; an image forming part for forming images for a plurality of pages in the set image formation order; a paper discharge mechanism for discharging the sheets used for forming the images for a plurality of pages to a post-processing device; and a conveyance control part for controlling the normal conveyance mechanism and the reversal conveyance mechanism so as to fold the sheets discharged from the image forming device, in the middle for the above number of times and to convey them to the post-processing part.