PURPOSE: To eliminate the wasteful consumption of electromotive force and to prevent erroneous operation, by allowing luminous intensity at the utilization time of a solar cell to correspond to electromotive force and performing the circuit control in the preparatory operation of electric machinery by said electromotive force.

CONSTITUTION: When a key input apparatus KEY is operated, the elapse time setting circuit of a microprocessor unit MPU is operated and, after a predetermined time was elapsed, the power source voltage of a solar cell SC is converted to a digital signal by an A/D converter and the electromotive force thereof is converted to a luminous intensity value by MPU. It is judged whether this luminous intensity value corresponds to necessary min. voltage for allowing MPU to operate normally and, if said value is equal to or more than the prescribed voltage, luminous intensity is displayed on a display apparatus DISP by the display control apparatus of MPU. If luminous intensity during use reaches a definite value or less, the A/D converter is stopped and the display of luminous intensity is also stopped.