PURPOSE: To easily check plural display devices in a short time.

CONSTITUTION: At the time of checking maintenance, a reference video signal is displayed from a reference signal generator 3 through a switch 2 and a distributor 4 at plural display devices 6(1), 6(2),... 6(n). Then, sensors 7(1), 7(2), and 7(n) convert the display luminance of each display face of the corresponding display device into a voltage, and transmits it to a fault detecting circuit 9. The fault detecting circuit 9 detects the display device of an abnormal system from the output voltage of the sensor, and the corresponding system is displayed at a monitor 10.