A spine formation device includes a sheet conveyer to transport a bundle of folded sheets, a pressing unit including a first pressing member movable according to a pressing load in a pressing direction perpendicular to the sheet conveyance direction to press opposed sides of the front end portion of the folded sheets and a second pressing member attached to the first pressing member, to press the front end portion of the folded sheets, and a spine formation member disposed at a predetermined distance from the pressing unit. The second moves in the sheet conveyance direction in conjunction with the first pressing member moving in the pressing direction and presses the folded portion of the bundle of sheets with a predetermined spine-forming load against a contact surface of the spine formation member, thereby forming a spine of the bundle of sheets.