A bookbinding system 1 shapes a booklet B in such a manner that a paper sheet bundle PS composed of plural text paper sheets P1 is bound by case binding using a cover paper sheet P2. In this case, a paper sheet having a size fitting within a region configured with a front surface PSF, a back surface PSB, and a spine PSa of the paper sheet bundle PS is used as the cover paper sheet P2. For example, vertically-long A4 paper sheets are used as the text paper sheets P1, and a horizontally-long A3 paper sheet is used as the cover paper sheet P2 to carry out case bookbinding. The cover paper sheet P2 does not protrude from the outer shape of the paper sheet bundle PS. Thus, it is not necessary to cut the cover paper sheet, and no cutting waste is generated.