A sheet processing apparatus includes an aligning member and a shift conveying unit. The aligning member is movable in a width direction perpendicular to a sheet conveying direction and presses a sheet stack loaded on a sheet processing tray so as to align the sheet stack in the width direction. The unit is provided on the upstream side of the tray and conveys a sheet, shifting the sheet in the width direction. Being shifted by the unit, sheets are loaded at first and second loading positions on the tray. When sheets are loaded at the first loading position, the aligning member is moved to a first standby position corresponding to the first loading position in advance. When sheets are loaded at the second loading position, the aligning member is moved in advance to a second standby position corresponding to the second loading position.