An oscillator, includes an amplifier circuit including a semiconductor element having a first constant potential as reference potential for a power supply voltage, a variable capacitance element, a piezoelectric resonator, and a capacitance circuit constituting a closed circuit with the piezoelectric resonator. The amplifier circuit and the variable capacitance element are connected in series to provide a series circuit. The capacitance circuit connects the capacitance elements in a plurality of numbers in series. A connecting midpoint of the series connection is connected to a circuit for the first constant potential. Two connecting midpoints other than the midpoint of the closed circuit are used as connecting points to connect the series circuit and the closed circuit in parallel.