A bookbinding device is disclosed to adhesively bind a back surface of a stack of pages to a center portion of a cover page, which includes a page supply unit (1), a thickness sensor (2), an adhesive applicator (3), a page conveyor unit (4), a cover supply unit (5), a press unit (8) and a cover folding unit (9). The cover supply unit includes a trimmer (6) for trimming a side edge portion of the cover sheet depending upon thickness of the page stack detected by the thickness sensor, and a positioning unit (7) for positioning the cover sheet such that a center line of the cover sheet which has been trimmed by the trimmer is aligned with a center of thickness of the page stack, at which position the page stack is adhesively bound to the cover sheet.