PURPOSE: To prevent the waste of energy due to the forgetting of the turning off of a power source by automatically interrupting the power source when the surrounding brightness of an equipment main body is less than a prescribed illuminance.

CONSTITUTION: An optical sensor part 17 converts the surrounding brightness into an electric signal, and transmits an interrupt signal through a bus line 2 to a CPU 1 when it is less than the preliminarily set illuminance. A power unit controller 15 constituted of a power source switch or a power source-on hold circuit or the like, interrupts the power source according to a control signal from a CPU 1, or sets a power down mode. Then, when room temperature is less than a normal value, whether or not an automatic power source interruption inhibiting signal is active is judged, the presence or absence of a key input is checked when it is inactive, whether or not any processing is being executed at present is discriminated when nothing happens even after the lapse of a waiting time, and the power source is interrupted when an idle state is obtained. Also, while any processing is being executed, the advisability of the recovery of the work is judged, and continuity or power interruption is selected.