A finisher includes an ejection port, a stacking tray, a light-emitting element, a light-receiving element, a communication section that performs data communication with an image forming apparatus and acquires a coverage rate of paper stacked on the stacking tray, and a paper-amount determination section that determines whether the amount of paper on the stacking tray exceeds a predetermined stack capacity of the stacking tray by comparing a predetermined threshold value with the amount of light received by the light-receiving element. The paper-amount determination section corrects the threshold value by using the coverage rate acquired by the communication section, compares the corrected threshold value with the amount of the light received by the light-receiving element, and determines that the amount of the paper stacked on the stacking tray exceeds the stack capacity of the stacking tray if the light amount is equal to or greater than the corrected threshold value.