PURPOSE: To prevent an oscillation frequency from changing against the change of an environmental temperature or a source voltage by providing an oscillator whose oscillation frequency is provided with characteristic having negative inclination against the change of the environmental temperature and that of the source voltage, and an oscillator with the characteristic having positive inclination.

CONSTITUTION: When at least either the environmental temperature or the source voltage VCC changes and the oscillation frequency f1 of the oscillator 1 is increased and simultaneously, the frequency f2 of the oscillator 2 is decreased, a phase comparator 3 outputs a signal of positive level. Next, a power source circuit 4 is operated so as to increase the voltage VCC to be supplied to the oscillators 1 and 2, and when the voltage VCC is increased, the f1 is lowered, and the f2 is raised, then, the frequencies f1 and f2 coincide with the oscillation frequency f0 of an output signal OUT. Also, when the frequency f1 is lowered and the frequency f2 is raised, the comparator 3 outputs a signal of negative level, and the circuit 4 is operated so as to decrease the voltage VCC to be supplied to the oscillators 1 and 2, and the frequency f1 is raised and the frequency f2 is lowered, then, the frequencies f1 and f2 coincide with the frequency f0. In such a way, it is possible to suppress the change of the oscillation frequency against that of the environmental temperature or the source voltage, and to always keep the frequency f0 of the output signal constant.