A stacker includes a plurality of sheet stacking units individually movable upward and downward. Sheets are selectively stacked onto either of the stacker trays. To remove the sheets stacked on one of the stacker trays, a door is opened. In a state where the door is open, the stacker trays moved by respective elevation units are movable only downward. In this manner, accidental upward movement of each of the stacker trays caused by a malfunction of a motor or the like is regulated, whereby damage to the stacker is prevented.