A sheet processing apparatus comprising: a sheet stacking unit on which sheets are accommodated and stacked; a binding unit that is movable in a direction perpendicular to a sheet conveying direction and binds a bundle of sheets; an aligning unit (51,51a,51b) configured to align the sheets stacked on the sheet stacking unit in the sheet conveying direction; and a moving unit configured to adjustably move, in the sheet conveying direction, the binding position by the binding unit with respect to an aligning position by the aligning unit; wherein: the sheet stacking unit is configured to accommodate sheets as a sheet bundle; characterised in that ligning members (51a,51b) of said aligning unit (51,51a,51b) are movable along a trailing end, in the sheet conveying direction, of the sheet bundle stacked on the sheet stacking unit and align the trailing end of the sheet bundle in the sheet conveying direction by abutting thereon; the binding unit is configured to move along said trailing end of the sheet bundle in the sheet conveying direction; and the moving unit is configured to move the aligning members (51a,51b) in the sheet conveying direction, wherein a distance from the trailing end of the sheet bundle in the sheet conveying direction to a binding position is adjusted by moving the aligning members (51a,51b) in the sheet conveying direction; the apparatus further comprising: a control unit configured to control, through the moving unit, positions to which the aligning members (51a,51b) are moved; wherein the control unit is configured to set the binding position based on a designation input from a user. a sheet stacking unit on which sheets are accommodated and stacked; a binding unit that is movable in a direction perpendicular to a sheet conveying direction and binds a bundle of sheets; an aligning unit (51,51a,51b) configured to align the sheets stacked on the sheet stacking unit in the sheet conveying direction; and a moving unit configured to adjustably move, in the sheet conveying direction, the binding position by the binding unit with respect to an aligning position by the aligning unit; wherein: the sheet stacking unit is configured to accommodate sheets as a sheet bundle; characterised in that ligning members (51a,51b) of said aligning unit (51,51a,51b) are movable along a trailing end, in the sheet conveying direction, of the sheet bundle stacked on the sheet stacking unit and align the trailing end of the sheet bundle in the sheet conveying direction by abutting thereon; the binding unit is configured to move along said trailing end of the sheet bundle in the sheet conveying direction; and the moving unit is configured to move the aligning members (51a,51b) in the sheet conveying direction, wherein the sheet stacking unit is configured to accommodate sheets as a sheet bundle; characterised in that ligning members (51a,51b) of said aligning unit (51,51a,51b) are movable along a trailing end, in the sheet conveying direction, of the sheet bundle stacked on the sheet stacking unit and align the trailing end of the sheet bundle in the sheet conveying direction by abutting thereon; the binding unit is configured to move along said trailing end of the sheet bundle in the sheet conveying direction; and the moving unit is configured to move the aligning members (51a,51b) in the sheet conveying direction, wherein a distance from the trailing end of the sheet bundle in the sheet conveying direction to a binding position is adjusted by moving the aligning members (51a,51b) in the sheet conveying direction; the apparatus further comprising: a control unit configured to control, through the moving unit, positions to which the aligning members (51a,51b) are moved; wherein the control unit is configured to set the binding position based on a designation input from a user. a control unit configured to control, through the moving unit, positions to which the aligning members (51a,51b) are moved; wherein the control unit is configured to set the binding position based on a designation input from a user.