A spine formation device includes a sheet conveyer, a clamping unit disposed downstream from the sheet conveyer in a sheet conveyance direction, for squeezing a folded portion of a bundle of folded sheets, a contact member against which the folded portion of the bundle is pressed, disposed downstream from the clamping unit, and a controller to cause the bundle of folded sheets to bulge by stopping the sheet conveyer after the bundle of folded sheets is transported a predetermined conveyance distance from a contact position between the contact member and the folded portion of the bundle and to cause the clamping unit to squeeze a bulging portion of the bundle created between the sheet conveyer and the contact member with the folded portion pressed against the contact member. The predetermined distance is set in accordance with a predetermined sheet-related variable.