A sheet processing apparatus comprising: a pair of first aligning members each including a stack portion (51a1; 51b1) to contact and support a trailing edge of the bundle to align the trailing edge of the bundle; a pair of second aligning members each extending in a sheet conveyance direction in which a bundle of sheets is transported and movable in a sheet width direction perpendicular to the sheet conveyance direction to align lateral sides of the bundle; a stapler movable in the sheet width direction to staple a trailing end portion of the bundle with staples at different positions in the sheet width direction after the bundle is aligned by the pair of first aligning members as well as the pair of second aligning members; a driving unit (50,55) to move the pair of first aligning members; and a controller communicably connected to the stapler and the driving unit (50,55) to move the first aligning members, wherein, the pair of first aligning members is movable in both the sheet conveyance direction and the sheet width direction perpendicular to the sheet conveyance direction, and the stapler is configured to staple the bundle at two positions (S1c) symmetrical relative to a centre position in the sheet width direction of the bundle, and the controller is configured to select either first positions inside the two positions (S1c) stapled symmetrically or second positions outside the two positions stapled symmetrically in accordance with a sheet size and positional data of stapled positions and set the pair of first aligning members at the selected positions to align the trailing edge of the bundle, when the stapler staples the bundle at two positions (S1c) symmetrical relative to a center position in the sheet width direction of the bundle, wherein the controller causes the driving unit (50,55) to move the first aligning members to either the first positions or the second positions before the bundle of sheets is aligned. a pair of first aligning members each including a stack portion (51a1; 51b1) to contact and support a trailing edge of the bundle to align the trailing edge of the bundle; a pair of second aligning members each extending in a sheet conveyance direction in which a bundle of sheets is transported and movable in a sheet width direction perpendicular to the sheet conveyance direction to align lateral sides of the bundle; a stapler movable in the sheet width direction to staple a trailing end portion of the bundle with staples at different positions in the sheet width direction after the bundle is aligned by the pair of first aligning members as well as the pair of second aligning members; a driving unit (50,55) to move the pair of first aligning members; and a controller communicably connected to the stapler and the driving unit (50,55) to move the first aligning members, wherein, the pair of first aligning members is movable in both the sheet conveyance direction and the sheet width direction perpendicular to the sheet conveyance direction, and the stapler is configured to staple the bundle at two positions (S1c) symmetrical relative to a centre position in the sheet width direction of the bundle, and the controller is configured to select either first positions inside the two positions (S1c) stapled symmetrically or second positions outside the two positions stapled symmetrically in accordance with a sheet size and positional data of stapled positions and set the pair of first aligning members at the selected positions to align the trailing edge of the bundle, when the stapler staples the bundle at two positions (S1c) symmetrical relative to a center position in the sheet width direction of the bundle, wherein the controller causes the driving unit (50,55) to move the first aligning members to either the first positions or the second positions before the bundle of sheets is aligned.