To provide an image forming system capable of binding a book by covering a bundle of paper sheets with a cover in a U shape and preventing useless output in bookbinding.

This image forming system can perform two modes including the first warning mode for determining warning when this image forming system determines that a side end part S1E of a paper sheet S1 and a side end part S2E of the cover S2 are not aligned in final form of a covered book S3 by regarding width of back cover as a predetermined value (for example, input width of back cover) after selecting the paper sheet S1 and the cover S2 used in bookbinding and the second warning mode for determining warning when this system determines that the side end part S1E of the paper sheet S1 and the side end part S2E of the cover S2 are not aligned in the final form of the covered book S3 by regarding width of the back cover as the measured thickness of a bundle of paper sheets S1 after measuring the thickness of the bundle of paper sheets S1. This system performs the first warning mode before starting output in bookbinding.

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