PURPOSE: To provide a paper post-treatment device which can bind papers without allowing an external force to concentrically work to one leg part of a stitching needle binding a bundle of papers even when the external force partially acts on the bundle of papers bound at one corner of one end thereof.

CONSTITUTION: A bundle of papers discharged on a bin 6 by a transfer unit 5 is caught by a clamper unit 2 and sent to a staple unit 1. The staple unit 1 moved parallel to a stitching position is driven by a stapler rotating motor, and rotated by a predetermined angle in a direction parallel to the surface of tone papers discharged onto the bin 6. Accordingly, the bundle of papers are bound at a corner of one end thereof while the stitching position of a staple is inclined to the end of the bundle of papers.