To provide a sheet processing apparatus capable of corner-forming without causing deformation of a binding needle, when corner-forming by press-contacting the spine of a booklet folded and bound by the binding needle.

A corner-forming processing apparatus 600 includes a lower holding plate 631 and an upper holding plate 633 holding a booklet S with a lower holding face 631a and an upper holding face 633a facing to each other; a first press-contact piece 650 and a second press-contact piece 651 having a first press-contact face 650p and a second press-contact face 651p press-contacting the spine T of the booklet S held by the lower holding plate 631 and the upper holding plate 633 to deform the spine T; and a moving means moving the first press-contact piece 650 and the second press-contact piece 651 along the spine T. Centers G of the first press-contact face 650p and the second press-contact face 651p are recessed in the press-contact direction P of the booklet S from both ends H of the first press-contact face 650p and the second press-contact face 651p.