PURPOSE: To compensate secular change in an oscillation frequency from a crystal oscillator digitally.

CONSTITUTION: Oscillation frequency signal from a crystal oscillator 10 is counted by a frequency counter 26 based on a clock from a satellite positioning device 12 and obtained frequency data are stored in an EEPROM 24 together with temperature data obtained by a temperature sensor 28. The temperature versus frequency characteristic of the crystal oscillator 10 is obtained based on data stored in the EEPROM 24 and the temperature versus frequency characteristic having been stored in the EEPROM 24 as a table are updated by new temperature versus frequency characteristic data prior to shipment. Since the new temperature versus frequency characteristic data reflects on the secular change in the oscillated frequency signal from the crystal oscillator 10, a system reference frequency obtained by a PLL 14 is controlled accurately to an object frequency independently of secular change by setting a frequency division ratio of a frequency divider 22 based on the temperature versus frequency characteristic data.