PURPOSE: To avoid the wear of batteries by a method wherein a display light is put out when it is clearly noticed from the outside that a power supply is closed.

CONSTITUTION: If a power saving function (p) is selected and a control circuit 20 judges that a display part 2 is in an open data with a detection switch 5, it is judged whether the display part 2 is in a backlighting state or not. That is, of the mode is the power saving mode, it is not noticed whether a power supply is applied to a main apparatus 21 or not only by watching the display part 2 which has no backlighting and a power-on lamp 3 is lit. If the mode is not the power saving mode, an operator can clearly notice that the power supply is applied to the main apparatus 21 by the backlighting of the display part 2. Therefore, the control circuit 20 operates a power-on lamp switch 22 to put out the power-on lamp 3. Thus, the waste lighting of the power-on lamp 3 is avoided and the power consumption can be saved.