To enable post-processing by accurately detecting a positional deviation in a width direction of paper while suppressing a decrease in the productivity as much as possible in a post-processing part of a paper post-processing device, and transferring the post-processing part to a proper position in the width direction of paper.

A paper post-processing device includes: a conveyance path for conveying paper; a post-processing part for subjecting the paper conveyed in the conveying path to post-processing; an edge face detecting part for detecting side edges of the paper conveyed to the post-processing part; a transfer part for transferring the post-processing part and the edge face detecting part in the width direction of the paper conveyed; and a control part for controlling the conveyance of the paper and the transfer while receiving a result of the detection by the edge face detecting part. The control part sets a starting position as a standard for the post-processing part and the edge face detecting part, performs the transfer from the starting position, determines the side edge position of the paper according to the result of the detection by the edge face detecting part, performs the transfer based on the side edge position to locate the post-processing part at a predetermined position, and, if an event to constitute a factor for a positional deviation in the width direction of the paper occurs, changes the starting position.