A device which aligns bundles of paper Q with a predetermined number of A4 paper PA4 as small size paper and A3 paper PA3 as large size papers formed by successively receiving paper P fed from image forming device 19, comprising a stacking unit 30 as a main retaining device which aligns and retains in a stacked condition paper P successively received from the image forming device 19, a paper detour retaining device 20 as a preliminary retaining device located directly upstream of the main retaining device stacking unit 30, which temporarily retains the paper P destined to the stacking unit 30, and a controller which controls feeding to the stacking unit 30 by temporarily retaining the A4 paper PA4 paper, and combining the retained A4 paper PA4 with the large size paper A3 paper PA3 before sending to the stacking unit 30 in an overlaid condition.