PURPOSE: To turn off a back light except when it is necessary for a liquid crystal display by turning off the back light when a timer means measures a time longer than a specific time and turning on the back light in the off state on detecting keying operation.
CONSTITUTION: A main control part 13 supplies a reset signal 18 to a time monitoring timer 17 every time code information is sent from a keyboard device 16 to reset its clocking operation. When clocking the preset specific time, e.g. three minutes, the time monitoring timer 17 supplies a clocking end signal 19 to the main control part 13 and sends out a control signal for indicating the off operation of the back light to a back light illumination control signal 13. Therefore, when keying on a keyboard device 16 is not performed for the specific time, the back light 12 is turned off automatically. Consequently, the back light can be turned off except when necessary to prolong the life of the back light and reduce the power consumption.