To provide a crystal oscillator which operates with the high stability of a frequency even just after the supply of power and is excellent in phase noise characteristic in a floor area.

With the output of OCXO2 as reference, a PLL control system detects a phase difference with the output of VCXO1 by means of a phase comparator 3, fetches its DC component by means of LPF5 and inputs it to VCXO7 as controlled voltage via a comparison switching unit 6. On starting a power source, the VCXO controlled voltage at the time of PLL control stored in a memory part 8 applied to the VCXO7 via the unit 6. With the outside of this VCXO7 as that of this oscillator, after the output frequency of the OCXO 2 is stabilized, the unit 6 is switched so as to constitute a PLL loop and the precision of the VCXO7 is pulled into the frequency precision of the OCXO2 to fetch the output of this VCXO7 as that of this oscillator.