To improve visibility of a display screen by preventing a backlight, which illuminates a liquid crystal display module from the rear side, from lacking in brightness at normal temperature.

In the case the ambient temperature T of the backlight detected with a temperature sensor 5 is lower than the specified upper limit temperature with guarantee of proper operation of the backlight, a host microcomputer 4 controls a driving current I of the backlight 1 to be higher than a driving current corresponding to the specified upper limit temperature with guarantee of proper operation of the backlight. For example, the host microcomputer 4 controls the driving current of the backlight so as to be the current with guarantee of proper operation corresponding to characteristics of the current with guarantee of proper operation in relation to the ambient temperature of the backlight.