To detect beforehand the life of a backlight part of a liquid crystal display device and to inform an operator of it.

Laser light 22 irradiates from a sensor part 7 so as to orthogonally intersect with light source light 21 irradiates a display part 5 from the backlight part 6. A sensor measuring part 8 measures the intensity of the laser light 22 after passing through the light source light 21 by quantity of heat. The measured quantity of heat Q is compared with the quantity of heat Q1 set as a value at a life time. When Q≥Q1, a life notice is performed to an operating system 10. The operating system receives that, and instructs for an application program 11 so as to display a life notice message on the display part 5 of the liquid crystal display device 2. The application program 11 receives that, and displays the' life notice message on the display part 5 of the liquid crystal display device 2, and prompts the operator to exchange the backlight.