A folded spine of a booklet held by a pair of opposing holding members is deformed by entering a pressing member into a gap between the pair of holding members and bringing the pressing member into pressure contact with the folded spine. In consequence, the folded spine is prevented from receiving an excessive pressing force, and a smooth surface with a width substantially equal to the thickness of the booklet is thus formed on the folded spine, whereby a booklet with a good appearance can be made.