A crystal oscillator which configures a voltage controlled oscillator having a variable oscillation frequency is also used as a crystal oscillator of an oscillator for generating a fixed frequency. Control input is integrated by an integrator, a phase of a frequency-divided signal obtained by dividing a fixed clock by a frequency divider is controlled by the integrated value to vary a frequency. Every time a phase-shifting amount at the time becomes to correspond to one clock, a frequency division ratio at dividing the clock is changed from a predetermined frequency division ratio for one time of frequency division to absorb the phase shifting amount for one clock, thereby operating as a voltage controlled oscillator. Thus, the voltage controlled oscillator having the frequency accuracy of a fixed clock is realized.
