To provide a binding processing device provided with a folding strengthening means for reducing swelling of a book by returning of folding of a saddle stitching folded book.

The binding processing device is provided with an accumulation part 17 for accumulating paper fed from a paper feeding device in every one sheet to a paper bundle for one volume; a stitching means 15 for stitching a center of the paper bundle; a folding means (folding knife 18, folding roller 14) for folding it into two; a book pressing means (pressing member 5, pressing member drive means 4, receiving plate 8); and a control means 19. When the control means 19 receives a final paper feeding signal or a signal indicating passing of the final paper, stitchinging/folding is started, pressing to the former book by the book pressing means is released to delivery it and thereafter, the saddle stitching folded book completed with stitching/folding is fed to start pressing. Thereby, the time required for accumulation of the paper is outstandingly utilized as a pressing time to strengthen the folding.

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