To cancel the fluctuation of oscillation frequency caused by the fluctuation of the power voltage by applying the voltage proportional to the drive voltage of an amplifier element used for feedback amplification to a voltage control type reactance element as the bias voltage.

This oscillator is provided with a feedback amplification transistor TR 1, a buffer amplification TR 2, a varactor diode 3, a λ/4 resonator 4, a capacitance correction capacitor 5, etc. The diode 3, resonator 4 and capacitor 5 construct an oscillation frequency setting circuit. A ground capacitor 14 is added between the anode of the diode 3 and a ground point, and the anode of the diode 3 is connected to the connection point between the bias resistances 17 and 18 which are connected in series to each other. Then the bias voltage V3 which is applied to the diode 3 is set at a level proportional to the drive voltage of the TR 1. As a result, the fluctuation of oscillation frequency due to the voltage fluctuation of a battery 28 can be eliminated.