PURPOSE: To correct the white balance of an adjacent screen, and to adjust the contrast of the whole multi-screen display device in the reference screen display device.

CONSTITUTION: A detecting signal is inserted into an overscan part of a video signal, a detecting signal is detected by a photodetector, and taking a luminance level of a projecting device as a reference, a luminance level of a projecting device 2 is corrected, and by the luminance level of the projecting device 2, a luminance level of a projecting device 1 is corrected. Also, by a common contrast adjusting part 7, the luminance level of the projecting device 3 is varied.