An spine formation device includes a sheet conveyer, a contact member including a first row of grooves provided in a contact surface thereof to house a projection projecting from a folded portion of a bundle of folded sheets, a driving unit to move the contact member, first and second sandwiching units to squeeze the bundle, a discharge unit, and a controller. The first row of grooves extends in parallel to the folded portion of the bundle and includes at least a first pair of grooves inclined in different directions with an interval therebetween varying in size with location of the grooves in a first direction perpendicular to a sheet conveyance direction. The contact member is moved to change the size of interval between the first pair of grooves at a position aligned with the projection projecting from the folded portion of the bundle of folded sheets.