<p>Horizontal check projections are continuously arranged from the upper part to the lower part of the inner wall surface of a case in which staple sheets are received in a layered manner. A check claw is provided on a staple press plate for pressing the staple sheets by a spring, and the check claw is engaged with a check projection of the case. The shapes of the check claw and the check projections are set so as to allow the staple press plate to descend but to prevent the plate from ascending. Further, the height of the tops of the check projections is reduced in a stepped or stepless manner from the upper part to the lower part of the inner wall surface of the case.</p>
