A sheet processing apparatus comprising: an accumulating section on which a sheet can be accumulated; a staple inserting section which is provided above the accumulating section; a staple receiving section which is provided so as to face the staple inserting section across a sheet bundle accumulated on the accumulating section; and a movement section which is suitable for moving an inserting unit or the accumulating section according to a thickness of the sheet bundle so that the inserting unit faces a predetermined position on an upmost sheet of the sheet bundle, wherein the staple inserting section includes the inserting unit which is suitable for inserting a staple into the sheet bundle by rotating toward the sheet bundle, wherein the accumulating section includes a saddle unit on which a mountain-folded sheet folded along a folding line at a middle portion can be accumulated so as to straddle the saddle unit, the staple receiving section is fixed inside the saddle unit so as to locate an upper part thereof at a top of the saddle unit, and the movement section is suitable for moving the inserting unit or the saddle unit in a direction orthogonal to the folding line along a horizontal plane according to the thickness of the sheet bundle so that the inserting unit faces the folding line on the upmost sheet of the sheet bundle accumulated on the saddle unit, wherein the sheet processing apparatus further comprises a storage section in which movement information regarding the thickness of the sheet bundle and a movement amount of the inserting unit or the accumulating section can be stored, wherein the movement section is suitable for determining the movement amount of the inserting unit or the accumulating section on a basis of the movement information stored in the storage section. an accumulating section on which a sheet can be accumulated; a staple inserting section which is provided above the accumulating section; a staple receiving section which is provided so as to face the staple inserting section across a sheet bundle accumulated on the accumulating section; and a movement section which is suitable for moving an inserting unit or the accumulating section according to a thickness of the sheet bundle so that the inserting unit faces a predetermined position on an upmost sheet of the sheet bundle, wherein the staple inserting section includes the inserting unit which is suitable for inserting a staple into the sheet bundle by rotating toward the sheet bundle, wherein the accumulating section includes a saddle unit on which a mountain-folded sheet folded along a folding line at a middle portion can be accumulated so as to straddle the saddle unit, the staple receiving section is fixed inside the saddle unit so as to locate an upper part thereof at a top of the saddle unit, and the movement section is suitable for moving the inserting unit or the saddle unit in a direction orthogonal to the folding line along a horizontal plane according to the thickness of the sheet bundle so that the inserting unit faces the folding line on the upmost sheet of the sheet bundle accumulated on the saddle unit, wherein wherein the staple inserting section includes the inserting unit which is suitable for inserting a staple into the sheet bundle by rotating toward the sheet bundle, wherein the accumulating section includes a saddle unit on which a mountain-folded sheet folded along a folding line at a middle portion can be accumulated so as to straddle the saddle unit, the staple receiving section is fixed inside the saddle unit so as to locate an upper part thereof at a top of the saddle unit, and the movement section is suitable for moving the inserting unit or the saddle unit in a direction orthogonal to the folding line along a horizontal plane according to the thickness of the sheet bundle so that the inserting unit faces the folding line on the upmost sheet of the sheet bundle accumulated on the saddle unit, wherein the sheet processing apparatus further comprises a storage section in which movement information regarding the thickness of the sheet bundle and a movement amount of the inserting unit or the accumulating section can be stored, wherein the movement section is suitable for determining the movement amount of the inserting unit or the accumulating section on a basis of the movement information stored in the storage section.