Abstract: In binding a cover sheet together with a sheet bundle, ever-accurate spine-creasing irrespective of bundle thickness is made possible. A sheet stacker stacks sheets into bundles, an adhesive applicator applies adhesive to a spine part of the sheet bundles, and a cover-sheet binder binds cover sheets together with the spine parts. The cover-sheet binder is constituted from a left-and-right pair of spine-creasing press members arranged free to shift between standby and spine-creasing positions; a shifter that reciprocates the spine-creasing press members between the standby and spine-creasing positions; and a controller. The controller functions to vary, in accordance with sheet bundle thickness, any of: (1) the standby position of the press members; (2) the start time of an operation whereby the press members are shifted from the standby to the spine-creasing position; or (3) the traveling speed of the press members in shifting from the standby to the spine-creasing position.