PURPOSE: To detect accurately the temperature of a crystal oscillator by constituting the system that an original frequency component is excluded, only the frequency changing component fluctuated by temprature is extracted and converted into a voltage so as to control a heating circuit controlling the temperature of a crystal oscillator.

CONSTITUTION: A required oscillating frequency is outputted from a terminal 5 of the oscillating circuit 4 by a crystal oscillator 1. A part of the utput is multiplied 6. The original frequency component is multiplied also together with a narrow frequency changing component fluctuated due to temperature at the multiplier 6. A low frequency filter 7 blocks the multiplied original frequency component and passes throug the multiplied narrow frequency changing component only. The passing low frequency component is converted into a voltage component corresponding to its frequency component at a frequency demodulator 8 and this voltage becomes a control signal of a heater control circuit 3 for heating. Thus, a heater power from terminals 9, 10 is controlled by a control circuit 3 and the heater power corresponding to the frequency fluctuation is applied to the heater 2 so as to detect accurately the temperature of the crystal oscillator 1.