PURPOSE: To judge the exchange time of an EL(electroluminescence) backlight beforehand and to early comply with the exchange by counting the lighting time of the EL backlight and comparing it with a previously set threshold.
CONSTITUTION: When the EL backlight 4 is turned on, time is read out from a calendar IC 5 and temporarily stored, and the time is read out also when the backlight 4 is turned off so as to calculate the difference of time. An EL backlight control task 1 sets the difference as the value of EL backlight lighting time, and an EL backlight lighting time counting processing part 2 adds the value of the EL backlight lighting time and a value stored in an EL backlight lighting time counting area 3 and stores the obtained value in the area 3. An operating system for a portable terminal 9 displays a maintenance message when the value stored in the area 3 becomes larger than the threshold at the time of starting a system. Thus, the accurate maintenance is accomplished.