To convey sheets from a post processing apparatus disposed on the far downstream side of an image forming apparatus to a post processing apparatus disposed on the upstream side thereof.

This image forming system 100 comprises an image forming apparatus A, the post processing apparatus PZ performing the process a punch and Z folder, and the post processing apparatus FS performing the processes of a paper inserter 40 or the like. Reverse conveying means formed of rollers drivingly rotated in the forward and reverse directions and drive motors are installed on the post processing apparatus PZ and the post processing apparatus FS. Also, a control means SC controlling the reverse conveying means is installed on the image forming apparatus A. As necessary, the sheets are conveyed from the post processing apparatus FS on the downstream side to the post processing apparatus PZ on the upstream side. By this, when the post processing is performed, the sheets are first conveyed to the post processing apparatus FS disposed on the downstream side and, after a high priority post processing is performed, they are disposed on the upstream side by the reverse conveying means, and then conveyed to the post processing apparatus PZ to perform a low priority post processing.