A piezoelectric oscillator is disclosed which falls under the category of an oscillator including a piezoelectric resonator, an amplifier, and a variable-capacitance element. The variable-capacitance element is a MOS construction type capacitance element, one terminal of that is fixed at a V voltage, and the other terminal of that is applied with a control voltage falling within a range whose intermediate value is the V voltage. As a result of this, a piezoelectric oscillator is realized which can vary its frequency over a wide range even without use of a minus power supply.