An image forming system including a binding section that generates a stack of sheets by accumulating a plurality of sheets and that binds by wrapping the stack of sheets with a cover sheet in the shape of a horizontal U, the image forming system comprising: a measurement section to measure the thickness of the stack of sheets; a spine cover width determining section to determine a spine cover width in the cover sheet based on the thickness of the stack of sheets measured by said measurement section; and an image forming section to form on the cover sheet a front cover image and a back cover image separating them by a spacing equal to said spine cover width.