Imaging-system bookbinding apparatus avoids cutting any folded sheets mixed into booklets it trims. Furnished with: an imaging unit; a sheet-folding unit that folds imaging-unit sheets; a stacking unit that stacks sheets from the sheet-folding or image-forming units; a cover-sheet binding unit that encases stacking-unit sheet bundles with, and binds them into, cover sheets; a trimming unit that trims true the bundle fore-edge; and a fold-position computing unit that determines where a sheet is pleated by the sheet-folding unit. The sheet-folding unit has a sheet conveyance path along which folded sheets are transported elsewhere than the stacking unit, while the fold-position computing unit in a first control mode transports to the stacking unit sheets folded over by the sheet-folding unit, and in a second control mode transports them from the sheet conveyance path elsewhere than the stacking unit.