To make an end of a bundle of transfer paper sheets between each job clear when bundles of the transfer paper sheets for a plurality of jobs are stacked and accumulated on a stack tray.

A paper post-processing device is provided with an intermediate tray 341 which stores a plurality of transfer paper sheets discharge from an image forming part temporarily and discharges a bundle of transfer paper sheets reaching a predetermined number of paper sheets to the outside, a stack tray 381 accumulating bundles of transfer paper sheets discharged from the intermediate tray 341 sequentially, a sorting means 343 which sorts the bundles of transfer paper sheets accumulated on the stack tray 381 in the right and left directions and is arranged on the intermediate tray 341, and a control means controlling the sorting means 343 so that sorting is done at all times between a bundle of transfer paper sheets of the last set of a first job and a bundle of transfer paper sheets of the first set of a second job when bundles of transfer paper sheets of a plurality of sets of the first job and bundles of transfer paper sheets of a plurality of sets of the second job are stacked and accumulated on the stack tray 381.