PURPOSE: To attain ease of temperature control, miniaturization and low power by switching plural crystal oscillators oscillated with a high stability at an overlapped temperature region at a detected temperature.

CONSTITUTION: A controller 4 impresses a power supply voltage Vcc to any of crystal oscillators 1, 2 at a temperature detected by a temperature sensor 3 so as to oscillate the crystal oscillator to which the voltage is impressed, stop the other oscillator, and the oscillated outputs are synthesized and extracted from an output terminal 5. The temperature range of each oscillator to be operated is overlapped partly in this case by switching the power supply voltage of the crystal oscillators so as to stabilize the switching operation even if the ambient temperature is fluctuated frequently and rapidly. Thus, the temperature control is attained easily and the miniaturization and low power are attained.