Disclosed is a sheet processing apparatus including an intermediate processing tray on which a sheet conveyed by a discharge roller is stacked, a proximal aligning plate and a distal aligning plate moving in a sheet width direction for aligning the sheet conveyed onto the intermediate processing tray, and a finisher controller for moving and controlling the proximal aligning plate and the distal aligning plate to align, when a sheet having a length in the sheet conveyance direction shorter than a predetermined length in the sheet conveyance direction is conveyed, the sheet at a position shifted nearer to a take-out port side A in the sheet width direction than a position in the sheet width direction at which a sheet having a length in the sheet conveyance direction equal to or more than the predetermined length in the sheet conveyance direction is aligned.