A sheet stacking apparatus for performing post-processing on sheets received from an image forming apparatus and discharging the processed sheets. A stacking tray has the received sheets stacked thereon. A CPU controls a tray lifting/lowering motor to drive the stacking tray to be lifted or lowered. The CPU determines a vertically moving state of the stacking tray. When the stacking tray is not lowered though the tray lifting/lowering motor is controlled to lower the stacking tray, the CPU notifies the image forming apparatus that a job should be suspended. After notifying, the CPU controls the tray lifting/lowering motor to lift the stacking tray. Then, when the stacking tray can be lifted, the CPU notifies the image forming apparatus that the job is to be resumed.