A spine formation device (3) includes a sheet conveyer (31), a clamping unit (32,325,326) disposed downstream from the sheet conveyer (31) in a sheet conveyance direction, for squeezing a folded portion of a bundle of folded sheets, a contact member (330) against which the folded portion of the bundle is pressed, disposed downstream from the clamping unit (32,325,326), and a controller (3-1) to cause the bundle of folded sheets to bulge by stopping the sheet conveyer (31) after the bundle of folded sheets is transported from a contact position between the contact member (330) and the folded portion of the bundle a predetermined conveyance distance set in accordance with a predetermined sheet-related variable, and to cause the clamping unit (32,325,326) to squeeze a bulging portion of the bundle created between the sheet conveyer (31) and the contact member (330) with the folded potion pressed against the contact member (330).