An image forming apparatus has a sheet stacking apparatus including a tray on which sheets are stacked, and an aligning member aligning the sheets stacked on the tray. The aligning member aligns sheets stacked, on a sheet previously stacked on the tray, with a displacement in an aligning direction of the aligning member with respect to the sheet previously stacked. An aligning operation of the aligning member is changed on the basis of whether or not the number of the sheets stacked on the sheets previously stacked is a predetermined number or less.