PURPOSE: To compensate erroneous stapling automatically and conduct optimum subsequent control by the causes of the erroneous stapling.

CONSTITUTION: This recording paper post-processing device, which stacks recording paper discharged from an image forming device and staples them with a stapler, is provided with a penetration detecting means 80 which detects if a staple 64 has penetrated through a bundle of recording paper, and a control means 92 which receives the no-penetration detection signal of the staple 64 by the penetration detecting means 80 and discriminating the generation of erroneous stapling. The control means 92 outputs a staple processing command again if it is discriminated that the erroneous stapling has occurred.