A bookbinding apparatus includes a collecting device, a bookbinding path, a bundle conveying device for transferring the sheet bundle to a paste applying position and then to a cover binding position, a paste applying device for applying an adhesive to a spine part of the sheet bundle, and a cover feeding path. A cover binding device binds the sheet bundle with the adhesive to the cover sheet, and a cover binding control device controls the cover binding device. The cover binding device includes a back folding press member and a back abutting plate member. The cover sheet is sandwiched between the back abutting plate member and the spine part of the sheet bundle with a small gap between the back abutting plate member and the spine part. The cover binding control device adjusts a size of the gap depending on a thickness of the cover sheet when folded back.