PURPOSE: To extend the duration of an operation by attaining power saving when the operation by a built-in battery is performed by detecting the use of an external power source and that of the built-in battery and lighting-out a backlight when the built-in battery is used.
CONSTITUTION: When a power is supplied from the external power source 1, a power input origin is immediately switched to the external power source 1 with a power source switching request signal 18 outputted from a known external power source detecting means, which reduces the consumption of the built-in battery 2. Also, when no power is supplied from the external power source 1, a built-in battery use confirmation signal 8 is outputted to a back light light-out circuit 9. In other words, a switch 20 is turned off when the built-in battery 2 is used, and the supply of the power source to a backlight power source 5 is interrupted, and no backlight 6 is turned on. However, it is possible to light the back light 6 even when the built-in battery is used by operating a switch 12 for backlight lighting.