Sheet stacking apparatus, sheet processing apparatus and image forming system

A sheet stacking apparatus comprising:

• a sheet discharge tray adapted to support on which discharged sheets, and to move upward and downward;
• an alignment section including a first regulating member and a second regulating member with which the alignment section aligns a side edge of a sheet discharged on the sheet discharge tray;
• a first drive section for moving the sheet discharge tray or the alignment section upward or downward;
• a second drive section for moving the first regulating member and the second regulating member in a conveyance width direction;
• and
• a control section for controlling the first drive section and the second drive section,
• wherein the control section controls the first drive section and the second drive section so as to:
• align the side edge of the sheet discharged on the sheet discharge tray on the basis of the position of the first regulating member, by fixing the position of the first regulating member and moving the second regulating member in the conveyance width direction;
• when a predetermined amount of the sheets are discharged on the sheet discharge tray, separate the sheets discharged on the discharge tray from the first regulating member and the second regulating member by moving the sheet discharge tray or the alignment section and, after that, move the first regulating member and the second regulating member in the conveyance width direction;
• and
• align the side edge of the sheets discharged on the sheet discharge tray by fixing the position of the second regulating member and moving the first regulating member in the conveyance width.