A sheet stacking apparatus having: a sheet discharge tray to move upward and downward; an alignment section including first and second regulating members to align a side edge of a sheet on the tray; a first drive section for moving the tray upward or downward; a second drive section for moving the regulating members in a conveyance width direction; and a control section for controlling the first and the second drive sections so as to: align the side edge of the sheet discharged on the tray on the basis of the position of the first regulating member; when a predetermined amount of the sheets are discharged on the tray, separate the sheets from the first regulating member and the second regulating member by moving the tray or the alignment section and, after that, move the regulating members in the conveyance width direction; and align the side edge of the sheets.