A sheet stacking apparatus configured to stack sheets, the sheet stacking apparatus comprising: upper discharge means configured to discharge a sheet; lower discharge means located vertically below the upper discharge means and configured to discharge a sheet; upper sheet stacking means independently elevatable corresponding to the upper discharge means and configured to stack thereon the sheet discharged from the upper discharge means; lower sheet stacking means independently elevatable corresponding to the lower discharge means and configured to stack thereon the sheet discharged from the lower discharge means; alignment means disposed between the upper and lower sheet stacking means (515, 516) and configured to align a position in a width direction perpendicular to a discharging direction of the sheet discharged on the lower sheet stacking means, control means configured to control the alignment means (517, 518), wherein the alignment means includes an alignment member configured to move in the width direction and align the position of the sheets stacked on the lower sheet stacking means in the width direction; wherein the alignment means further includes moving means configured to move said alignment member downward to an alignable position in which the sheets can be aligned by the movement of the alignment member in the width direction when the sheets are aligned, and to move the alignment member upward from the alignable position to a retracting position after the sheets are aligned, wherein, when the sheets stacked on the lower sheet stacking means are to be removed, the control means controls the moving means so that the alignment member is moved to the retracting position above said alignable position, and characterized in that when the alignment member is moved to the retracting position, the control means controls the moving means so that the retracting position is changed according to the position of the upper sheet stacking means detected by detection means configured to detect a position of said upper sheet stacking means, so as to prevent the alignment member from contacting the upper sheet stacking means. upper discharge means configured to discharge a sheet; lower discharge means located vertically below the upper discharge means and configured to discharge a sheet; upper sheet stacking means independently elevatable corresponding to the upper discharge means and configured to stack thereon the sheet discharged from the upper discharge means; lower sheet stacking means independently elevatable corresponding to the lower discharge means and configured to stack thereon the sheet discharged from the lower discharge means; alignment means disposed between the upper and lower sheet stacking means (515, 516) and configured to align a position in a width direction perpendicular to a discharging direction of the sheet discharged on the lower sheet stacking means, control means configured to control the alignment means (517, 518), wherein the alignment means includes an alignment member configured to move in the width direction and align the position of the sheets stacked on the lower sheet stacking means in the width direction; wherein the alignment means further includes moving means configured to move said alignment member downward to an alignable position in which the sheets can be aligned by the movement of the alignment member in the width direction when the sheets are aligned, and to move the alignment member upward from the alignable position to a retracting position after the sheets are aligned, wherein, when the sheets stacked on the lower sheet stacking means are to be removed, the control means controls the moving means so that the alignment member is moved to the retracting position above said alignable position, and characterized in that when the alignment member is moved to the retracting position, the control means controls the moving means so that the retracting position is changed according to the position of the upper sheet stacking means detected by detection means configured to detect a position of said upper sheet stacking means, so as to prevent the alignment member from contacting the upper sheet stacking means.