PURPOSE: To prevent the power consumption of a primary battery and to save the energy of a power by switching a power supply changeover switch to supply a power switch signal to the power switch terminal of a CPU via the other contact of the power supply changeover switch while the primary battery is working.

CONSTITUTION: In the dark, the switch button of a power supply changeover switch 8 is set at the right end position and the contacts 8a and 8b are closed. Thus a primary battery circuit consisting of a primary battery 3 is connected in parallel to a solar battery circuit consisting of a solar battery 2. Under such conditions, a CPU 14, etc., are driven with the battery 3 used as a drive power supply. Then the display contents which are inputted from a key part 9 and successively shown at a liquid crystal display part 4 are held. However, the potential of the cathode side of a diode 12 is inputted to a power switch terminal 16 of the CPU 14 as a signal when the contact 8b is closed. Thus an APO function operates and the supply of power voltage is automatically cut by the CPU 14 to the part 4 if the key operations are not carried out for a fixed time, e.g. 7 minutes or so.