PURPOSE: To decrease the change in the oscillated frequency by using an output of a voltage converter so as to control a voltage controlled piezoelectric oscillator thereby averaging an output of the controlled voltage of the oscillator.

CONSTITUTION: A temperature compensation digital code corresponding to a digital code of an analog/digital converter 2 being e temperature address signal is stored in a memory circuit 3 to compensate the frequency temperature characteristic of the voltage controlled oscillator 5 in advance, and when the ambient temperature is changed, the address signal is also changed and a temperature compensation code is read out of the memory circuit 3. The temperature compensation digital code read in this way is fed to a digital/analog converter 4, where the signal is converted into a stepwise analog signal, which is fed to a voltage converter 6. Thus, the stepwise analog signal is averaged by the voltage converter 6 and its output is fed to the voltage controlled piezoelectric oscillator 5 as a control voltage.