PURPOSE: To bring an oscillating frequency to a prescribed frequency accurately by controlling a temperature of a crystal resonator based on a difference between a frequency n times the frequency of the fundamental wave vibration and the frequency of n-th order overtone vibration.

CONSTITUTION: Temperature is detected with high accuracy and high sensitivity by selecting a difference between a frequency n times the frequency of the fundamental vibration at a reference temperature and a frequency of n-th order overtone vibration to be a frequency of several kNz - several hundreds kHz being convenient for the signal processing such as amplification, multiplication and D/A conversion. The temperature of the crystal resonator is controlled depending on the difference between the detected temperature and the reference temperature. Thus, the vibration frequency obtained from the crystal resonator is made stable to a prescribed value.