SHEET STACKING DEVICE AND SHEET PROCESSING DEVICE, AND IMAGE FORMING APPARATUS

PROBLEM TO BE SOLVED: To provide a sheet stacking device and a sheet processing device having improved visibility and extractability of sheets to be discharged, and an image forming apparatus including the same.SOLUTION: The image forming apparatus includes an intermediate processing tray 540, a sheet processing unit including a front-side alignment plate 541 and a back-side alignment plate 542 that move in a sheet width direction crossing a sheet conveyance direction to align sheets conveyed to the intermediate processing tray 540 in the sheet width direction, a stacking tray 504 on which the sheets processed by the sheet processing unit are stacked and from which the stacked sheets are extracted at its one end in the sheet width direction, and a finisher control unit for performing movement control of the front-side alignment plate 541 and the back-side alignment plate 542 so that the sheets having a predetermined length or more in the sheet conveyance direction are aligned at a first position in the sheet width direction when stacked on the stacking tray 504 and the sheets having a length less than the predetermined length in the sheet conveyance direction are aligned at a second position closer to the one end than the first position when stacked on the stacking tray 504.