PURPOSE: To compensate automatically aging changes in an output frequency of an oscillator by storing in advance the relation between the oscillating frequency and the logarithmic time axis changed linearly and measuring the operating time of the oscillator to compensate the frequency.

CONSTITUTION: The relation between the oscillating frequency of the oscillator and the logarithmic time changed linearly is utilized and the relation is stored in a memory of a controller 4 in advance. Then the output frequency of a variable synthesizer 2 is counted by a time counter circuit 3 to measure the operating time of the oscillator 1. The variable synthesizer 2 driven by the oscillator 1 is controlled automatically based on the operating time and the said storage content to change the output frequency of the variable synthesizer 2 thereby compensating the frequency.