PURPOSE: To obtain the temperature frequency characteristic with high stability by digitizing an output from a temperature sensor and converting a compensated digital data into an anlog data so as to control a crystal oscillator.

CONSTITUTION: An input of the temperature sensor 2 is written in an address of a PROM4 through an A/D converter 3 so as to read the compensated digital data Cn stored therein, a temperature compensation DC voltage Vo is applied to the crystal oscillator 1 through a D/A converter 5 so as to obtain the temperature frequency characteristic with high stability. Then a frequency comparator 6 of a data processing unit 10 mounted removably outputs a frequency difference between an output frequency and the stadard frequency, and a microcomputer 7 outputs the compensated digital data Cn and writes it in the PROM4.