To provide a crystal oscillator that exhibits an increased oscillator frequency variation amount in accordance with an externally supplied control voltage and that provides a frequency variation exhibiting an improved linearity.

The crystal oscillator is characterized in to include a frequency adjusting circuit configured by use of MOS voltage variable capacitance elements, the frequency adjusting circuit has the two MOS voltage variable capacitor elements, the back gate terminal of each MOS voltage variable capacitance element receives a control DC voltage via a respective one of level shift circuits, the voltage shift amount of one of the level shift circuits is established such that the corresponding MOS voltage variable capacitance element operates in a range where the C-V characteristic is linear in a range lower than the center voltage of the control DC voltage, and the voltage shift amount of the other of the level shift circuits is set such that the corresponding MOS voltage variable capacitance element operates in a range where the C-V characteristic is linear in a range higher than the center voltage of the control DC voltage.