PURPOSE: To compensate a temperature extending over a very wide range, by changing the effective electrode area of a piezoelectric oscillator by a duty pulse to be varied.

CONSTITUTION: The oxcillation frequency of a ring oscillator RG consisting of odd number of C-MOS invertors is linearly varied by a temperature. This output and the output of the frequency dividing circuit D are provided to the mixer MR, and when the difference of both the frequencies is converted to a digital data by the data converting circuit DT and it is provided to the counter CT, a pulse of duty corresponding to an output data of the converting circuit DT is generated from the counter CT. The switching circuit S is opened and closed by this duty pulse, and the effective electrode area of the quartz oscillator A is changed. Namely, opening and closing of the switching circuit S is controlled in accordance with a temperature, and a temperature of the oscillation frequency is compensated. In this way, a temperature is compensated extending over a very wide range.