PURPOSE: To decrease the fluctuation of an oscillated frequency due to the fluctuation of a power voltage by providing a resistor determining the time constant of the oscillation circuit and whose resistance is changed by a prescribed condition and a means controlling a resistance in relation to the fluctuation of the power voltage.

CONSTITUTION: When a light is projected onto a solar battery 13 and the illuminance is increased, the power voltage of a computer 10 gets higher and the oscillated frequency F gets higher. A current flows to a light emitting diode 18 with a voltage v3 thereacross, the light emitting diode 18 is lighted and heated and the temperature of the oscillation resistor 16 rises. Since a temperature compensation resistor having a positive temperaturte coefficient is used for the oscillation resistor 16, the characteristic of the power voltage V versus oscillated frequency of the computer 10 is folded nearly in a chevron shape at a coundary of the voltage v3 at which the light emitting diode 18 starts lighting and heating. Thus, the frequency fluctuation is suppressed by ΔF.