To prevent reduction in luminance, to adjust the luminance by preventing a fluctuation in the luminance, and to secure an integral value of luminance adjusting display time even when a power source 1s cut off by correcting the luminance by seizing reduction in the luminance by measuring the integral value of the display time.

Reduction in luminance by displaying display devices for many hours is corrected by constituting so that display luminance of the display devices changes according to an integral value of display time. The luminance of plural display devices is constituted in the same by respectively independently changing the display luminance of the display devices according to the integral value of the respective display time. Therefore, the display luminance of the plural display devices different in the integral value of the display time is set in the same to form the devices excellent in visibility and operability. Integral value data of current-carrying time added by a microcomputer 17 is always sent to a device 28 in a constant period by holding data even when a power source 21 is cut off by the nonvolatile storage device 28 in measuring display time. Therefore, luminance correcting reliability can be improved by forming the display devices always bright/easy-to-see over a long period.