A folded back portion flattening device includes a gripper and a presser. The gripper grips a sheet bundle formed into a booklet with the folded back portion protruding from the gripper. The presser presses the folded back portion of the sheet bundle gripped by the gripper, and includes pressing rollers which have pressing rotational surfaces and which are disposed along the folded back portion. The pressing rotational surfaces press the folded back portion. The pressing rollers are disposed so that the rotational pressing surface of a second pressing roller, which presses the folded back portion after the rotational pressing surface of a first pressing roller has pressed the folded back portion, presses deeper into an edge of the sheet bundle than the rotational pressing surface of the first pressing roller. At least one of the gripper and presser is movable in a direction parallel to the folded back portion.