PURPOSE: To make it possible to adjust the frequency of quartz oscillators easily and to improve the yield, by measuring the frequency variation rate of an oscillation circuit until a temperature which is almost equal to the middle of peak temperature of high and low temperatures quartz oscillators and by obtaining the value of capacitor necessary to adjust the frequency of oscillation circuit.

CONSTITUTION: A quartz oscillator having a 33°C peak temperature, the oscillating frequency of single body at the normal temperature of which is known, and another quartz oscillator, only the oscillating frequency of which is known, are connected to each other in parallel. A frequency trimming is made on them by changing the capacity of CMOS invertor 1 and capacitor CG, etc., and the oscillating frequency of the oscillation circuit at the normal temperature is adjusted to become a prescribed frequency. Then the frequency variation rate of oscillation circuit is measured until a temperature which corresponds to about the middle of peak temperature of both quartz oscillators, for instance -10°C, and the value of capacitor Cco necessary to adjust the frequency of oscillation circuit is obtained from the value. Therefore, frequency adjustment of the quartz oscillator can be made easily and the yield can be improved.