An image forming system comprising an image forming device and a bookbinding device coupled to the image forming device, the image forming device being configured to form an image on body sheets and the bookbinding device being configured to wrap, in a square shape, a stack of body sheets having the image formed thereon with a cover, the bookbinding device comprising: a processor, a storage section coupled to the processor, a sheet stack holding section for holding the stack of body sheets, a cover joining section that joins the cover to the stack of body sheets stored in the sheet stack holding section, and a cover feeding section configured to convey the cover to the cover joining section for joining the cover to the stack of body sheets, the bookbinding device being configured to store printing conditions in the storage section, the printing conditions including adjustment information related to a relative position between the cover and the stack of body sheets, wherein the joining section includes a conveying roller and a motor to convey the cover on the joining section, the motor being controlled by the processor to stop the cover at a predetermined position in accordance with the adjustment information to control the relative position between the cover and the stack of body sheets, and the joining section further comprises a detector which detects the edge of the cover conveyed to the joining section, the relative position between the cover and the stack of body sheets being controlled based on detection of the edge of the cover by the detector and the information related to the relative position stored in the storage section, wherein the relative position is between an edge of the cover corresponding to an edge of the stack of body sheets that will become a fore edge when the cover is joined with the body sheets, and the edge of the stack of body sheets that will become the fore edge, and the relative position is controlled by controlling the predetermined position of the cover conveyed on the joining section. a processor, a storage section coupled to the processor, a sheet stack holding section for holding the stack of body sheets, a cover joining section that joins the cover to the stack of body sheets stored in the sheet stack holding section, and a cover feeding section configured to convey the cover to the cover joining section for joining the cover to the stack of body sheets, the bookbinding device being configured to store printing conditions in the storage section, the printing conditions including adjustment information related to a relative position between the cover and the stack of body sheets, wherein the joining section includes a conveying roller and a motor to convey the cover on the joining section, the motor being controlled by the processor to stop the cover at a predetermined position in accordance with the adjustment information to control the relative position between the cover and the stack of body sheets, and the joining section further comprises a detector which detects the edge of the cover conveyed to the joining section, the relative position between the cover and the stack of body sheets being controlled based on detection of the edge of the cover by the detector and the information related to the relative position stored in the storage section, wherein the relative position is between an edge of the cover corresponding to an edge of the stack of body sheets that will become a fore edge when the cover is joined with the body sheets, and the edge of the stack of body sheets that will become the fore edge, and the relative position is controlled by controlling the predetermined position of the cover conveyed on the joining section.