PURPOSE: To enable compensating of temperature in high accuracy by using a signal front from first oscillator at delay side normally and conducting an operation of using a signal from second oscillator at lead side for predetermined period in number proportional to the differential frequency.

CONSTITUTION: A detector 13 detects the moment that the phase difference between a first oscillator A generating slightly lower frequency and a second oscillator B generating slightly higher frequency at different top temperature in temperature characteristics from the first crystal vibrator 1 to feed the number of beats during certain period T1 to a controller 12 so as to move a forcible synchronizer 11 during T2 seconds from the moment of the beat to thereby oscillate the oscillator A at frequency f2. In this meantime, the frequencies of both the output signals P1 and P2 are equal to f2 to detect no beat. Therefore, the controller 12 receives the signal P4 of a memory 14 as beat. In the residual period the oscillator A oscillator A oscillates at frequency f1 so that the mean frequency becomes constant.