A sheet stacking apparatus comprising: a first discharge unit configured to discharge a sheet; a first stacking unit on which the sheet discharged by the first discharge unit is stacked; a second discharge unit disposed below the first discharge unit and configured to discharge a sheet; a second stacking unit on which the sheet discharged by the second discharge unit is stacked; a first elevating unit configured to elevate the first stacking unit; a second elevating unit configured to elevate the second stacking unit; said first and second stacking units (515, 516) being independently elevatable; an alignment member disposed between said first and second stacking units (515, 516) and configured to move in the width direction perpendicular to a sheet discharging direction, and to align a position of the sheets stacked on the second stacking unit in the width direction perpendicular to a sheet discharging direction; a control unit configured to control the alignment member; and a moving unit (M3, 538, 536, 535) configured to move the alignment member downward to an alignment position where the sheets are to be aligned by the movement of the alignment member in the width direction perpendicular to a sheet discharging direction, and after the sheets are aligned, upward to a first position above the alignment position and to a second position between the alignment position and the first position; characterized by further comprising: a detecting unit (S8, S6) configured to detect a position of the first stacking unit in a vertical direction; wherein the control unit is configured to control the moving unit (M3, 538, 536, 535) to move the alignment member to the second position in a case where the detecting unit (S8, S6) detects that the first stacking unit is positioned below a predetermined position, and wherein the control unit is configured to control the moving unit (M3, 538, 536, 535) to move the alignment member to the first position in a case where the detecting unit (S8, S6) detects that the first stacking unit is positioned above the predetermined position, wherein in said both cases the alignment member does not contact the first stacking unit. a first discharge unit configured to discharge a sheet; a first stacking unit on which the sheet discharged by the first discharge unit is stacked; a second discharge unit disposed below the first discharge unit and configured to discharge a sheet; a second stacking unit on which the sheet discharged by the second discharge unit is stacked; a first elevating unit configured to elevate the first stacking unit; a second elevating unit configured to elevate the second stacking unit; said first and second stacking units (515, 516) being independently elevatable; an alignment member disposed between said first and second stacking units (515, 516) and configured to move in the width direction perpendicular to a sheet discharging direction, and to align a position of the sheets stacked on the second stacking unit in the width direction perpendicular to a sheet discharging direction; a control unit configured to control the alignment member; and a moving unit (M3, 538, 536, 535) configured to move the alignment member downward to an alignment position where the sheets are to be aligned by the movement of the alignment member in the width direction perpendicular to a sheet discharging direction, and after the sheets are aligned, upward to a first position above the alignment position and to a second position between the alignment position and the first position; characterized by further comprising: a detecting unit (S8, S6) configured to detect a position of the first stacking unit in a vertical direction; wherein the control unit is configured to control the moving unit (M3, 538, 536, 535) to move the alignment member to the second position in a case where the detecting unit (S8, S6) detects that the first stacking unit is positioned below a predetermined position, and wherein the control unit is configured to control the moving unit (M3, 538, 536, 535) to move the alignment member to the first position in a case where the detecting unit (S8, S6) detects that the first stacking unit is positioned above the predetermined position, wherein in said both cases the alignment member does not contact the first stacking unit. a detecting unit (S8, S6) configured to detect a position of the first stacking unit in a vertical direction; wherein the control unit is configured to control the moving unit (M3, 538, 536, 535) to move the alignment member to the second position in a case where the detecting unit (S8, S6) detects that the first stacking unit is positioned below a predetermined position, and wherein the control unit is configured to control the moving unit (M3, 538, 536, 535) to move the alignment member to the first position in a case where the detecting unit (S8, S6) detects that the first stacking unit is positioned above the predetermined position, wherein in said both cases the alignment member does not contact the first stacking unit.