To improve folding quality by suppressing flexure formed when folding paper or a paper sheaf.
A paper post-processing device FR includes a pair of carrier path guide plates 90 to which paper P is carried; a paper folding plate 72 folding the center of the paper P or a paper sheaf; and a plate-like folding blade with its tip abutting on the surface of the paper P located between the carrier path guide plates 90 to feed the paper P or the paper sheaf into a carrier path 75 including the paper folding plate 72. Since the device FR is provided with a space changing means for changing a space between the pair of carrier path guide plates 90, the space between carrier paths is narrowed when folding the paper P or the paper sheaf (S109) and widened when stacking and aligning the paper P or the paper sheaf (S102).
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