A sheet stacking apparatus has a shutter mechanism, having a shutter, for controlling the stacking and dropping operations of sheets to be stacked in a stacking safe. When a sensor detects that the sheets to be stacked on the shutter have been stacked to reached a predetermined height, the shutter is moved downward to reserve a space in the vicinity of a sheet loading port, and is opened to cause the sheets to drop onto the lower portion of the stacking safe. Then, the shutter is moved upward to the home position to stack the sheets thereon. When these shutter downward and upward movements are repeatedly performed, stacking can be performed stably, and the sheets discharged from the lower bottom of the stacking safe can be collected in the upper portion of the stacking safe through a communication path outside the stacking safe, thereby continuously performing a series of sheets loading and discharge operations.