A neutralization of an equivalent parallel capacitor of a piezoelectric resonator is realized to obtain a stable activation of oscillation and secure a large frequency variation. A crystal resonator is connected between an input and output terminals of an inverting amplifier to form a Colpitts-type oscillator circuit, an input terminal of another inverting amplifier is connected to the output terminal through a capacitor and the output terminal is connected to the input terminal through another capacitor to form a Miller capacitor circuit for electrically neutralizing a parallel capacitor existing equivalently between both sides of the crystal resonator.