A sheet processing apparatus includes a saddle unit on which a mountain-folded sheet folded along a folding line is accumulated so as to straddle the saddle unit, a staple inserting section disposed above the saddle unit, and a staple receiving section accommodated in the saddle unit. The staple inserting section includes an inserting unit which inserts a staple into the folding lines of the sheets, and holding members disposed on both sides of the inserting unit in a direction parallel to the folding lines of the sheets. The staple receiving section includes a staple receiving unit which receives and bends the tips of the staple inserted by the inserting unit, and supporting members disposed on both sides of the staple receiving unit at positions opposite to the holding members.