Image-forming system and bookbinding method, for collating and stacking sheets and binding together their spine-portion edges, that allows easy insertion of foldout leaves in the course of the bookbinding processes. Bookbinding-finishing method for collating printed sheets into bundles and binding together the spine-portion edges includes: an image-forming step of sequentially forming images on a plurality of sheets, based on predetermined image data; a fold-process step of folding the printed sheets from the image-forming step; a stacking step of collating into bundles the printed sheets from the image-forming step and/or the sheets folded in the fold-process step; a bookbinding step of binding together the spine-portion edges of the sheet bundles collated in the stacking step; and a trim-finishing step of trimming true at least the fore-edge of the sheet bundles bound in the bookbinding step.