A pressing unit includes a pressure roller that slides on the folded side while rotating, an elastic biasing unit that presses the pressure roller in a thickness direction of the stack of sheets, and a driving unit that slides the pressure roller in a direction substantially perpendicular to a conveying direction of the stack of sheets. A lifting unit, when the pressure roller slides to a first position, temporarily lifts up the pressure roller, and when lifted-up pressure roller slides to a second position, lifts the lifted-up pressure roller down onto the folded side.