PURPOSE: To obtain highly accurate and highly stable frequency simultaneously with turning on a power supply by using the output of a temperature compensating crystal oscillator until the stabilization of oscillation frequency of a highly stable crystal oscillator, and after stabilization, switching said output.

CONSTITUTION: When the output of a thermostatic oven controller OVEN CONTROL reaches a prescribed value, an alteration switch SW2 inserted between an oscillation circuit of the highly stable crystal oscillator OCXO and a driving power supply for said circuit is closed to oscillate the OCXO and an alteration switch SW3 inserted between an oscillation circuit of the temperature compensating crystal oscillator TCXO and its driving power supply is opened by using the output signal of the OCXO. Thus, only the TCXO is driven until the stabilization of the oscillation frequency of the OCXO, and after stabilizing the frequency of the OCXO, the TCXO stops driving and only the output of the OCXO can be used.