To provide a paper post-processing device for carrying out crease formation on paper as preprocessing for stacking and folding stacked paper, while achieving adequate crease formation corresponding to the number of sheets of the stacked paper.

A crease forming device 19 provided on a folding processing passage C of the paper post-processing device 1 has a plurality of crease forming blades 191. The plurality of crease forming blades 191 form two creases on the folding position of paper S. A space between the two creases is determined in accordance with a stacking order of the paper S. The stacking order relates to whether the paper is outside or inside the stacked paper when becoming a booklet. Two crease forming blades 191 are selected out of the plurality of crease forming blades 191 to produce the determined crease space. The selected crease forming blades 191 form creases on the paper S.