PURPOSE: To prevent an edge of a jammed paper from being folded by providing a couple of trays which are movable on a plane periodically at right angles to the conveyance direction to an intermediate stack part, and controlling the operation of the trays with a signal from a paper sensor provided to a discharge slot associatively with the conveyance of the paper.

CONSTITUTION: Sheets of paper which are received in a finishing device 3 one after another as copying operation is started pass through an inversion part 8 to operate the paper sensor 23 and are stacked at a tray part, but when the sensor 23 with a butterfly is turned on by paper, the trays 9a and 9b move to backward positions by about 5cm from a specified size position and move again to the specified paper size position about t1 seconds after the sensor 23 is turned off in response to the completion of the passage of the paper; and both edges of copying paper conveyed and stacked newly are clamped by tray fences 9c and 9d to perform paper arranging operation plural times. Consequently, the sheets of paper are stacked regularly.