A creasing device includes a feeding mechanism, a creasing mechanism, sensors, and an aligning mechanism. The feeding mechanism conveys paper on which an image has been formed in a post-processing apparatus main body direction. The creasing mechanism makes a crease on the paper. The sensors detect a deviation of the paper with respect to the feeding direction. The aligning mechanism, based on an amount of deviation in a result of a detection by the sensors, causes the creasing mechanism to rotate in such a way that the creasing mechanism is aligned parallel to a crease direction of the paper.
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