In a sheet stacking apparatus, a pair of side members includes a pair of lower side members which is provided with an approximately same distance therebetween as a width of a planographic printing plate and limits a movement in the planographic printing plate width direction; and a pair of upper side members which are provided above the pair of lower side members, each of which is provided with a movable section movable in the planographic printing plate width direction, and on which tapered surfaces are formed facing each other in a generally V shape. The distance between the pair of upper side members is controlled by driving the movable sections of the pair of upper side members so as to prevent quality degradation caused by a thrown-in sheet scratching a stacked sheet.