Patent ID: 8770571

Claim:
A sheet stacking apparatus comprising: a discharge unit configured to discharge a sheet; a stacking tray on which the sheet to be discharged by the discharge unit is stacked; an alignment unit configured to align the sheet stacked on the stacking tray in a width direction which is orthogonal to a direction in which the sheet is discharged, wherein the alignment unit includes first and second alignment members configured to move in the width direction and to come into contact with side ends in the width direction of the sheet stacked on the stacking tray to align the sheet, and wherein one of the first and second alignment members is placed on a first sheet in a case where a subsequent second sheet to be shifted in the width direction and stacked on the first sheet stacked on the stacking tray is aligned; and a control unit configured to prohibit, if a length of the second sheet in the width direction is different from a length of the first sheet in a case where the second sheet is shifted in the width direction and stacked on the first sheet on the stacking tray, an alignment operation by the alignment unit on the second sheet.