PURPOSE: To obtain a stable oscillated output with a wide variable range by connecting a capacitor in parallel with a series resonance circuit connected to a abse and connecting a series circuit comprising with a capacitor and a resistor in parallel with a resistor connected to an emitter.

CONSTITUTION: A frequency-phase characteristic of a series resonance circuit 31 is changed from curve 36 to curve 37 as shown in Fig. and the slope of the curve at a series resonance frequency f0 is made steep by connecting a capacitor 33 in parallel with a series resonance circuit constituted by connecting an inductor 8 to a varactor diode 7. Moreover, the reflection coefficient γ at the emitter side is changed by allowing a resistor 34 and a capacitor 35 to be connected to an emitter terminal 11 constituting an amplifier 32 for oscillation, and the frequency-phase characteristic of the reflection coefficient γ viewed from the base side is changed from curve 38 to curve 39 in Fig. and made gentle. Thus, the oscillated frequency fr approaches the f0, by being changed from fr1 to fr2 and the variable range is made wide. Furthermore, parasitic oscillation is also eliminated.