PURPOSE: To increase the variable width of frequency by inserting a parallel circuit of a variable capacity diode and an inductance in series of a piezoelectric resonator.

CONSTITUTION: A parallel circuit is formed by connecting a variable capacity diode Cj in series to a piezoelectric resonator 1 together with an inductance Ls connected in parallel to the diode Cj. Thus the adverse bias voltage impressed to the diode Cj is varied to secure the function of a voltage controlled oscillator. Here a constant is set so that the parallel resonance frequency of the diode Cj and an inductance L3 is higher than the anti-resonance frequency of the resonator 1. Then the function of an inductance L is secured in a using frequency band. Thus the variable width of frequency is increased.