PURPOSE: To obtain a microwave oscillator small in size and easy to adjust even at a low frequency, by providing conductor bar supported with an insulator in the internal conductor of a coaxial dielectric oscillator formed with a conductor film, and set at the same potential as that of an external conductor.

CONSTITUTION: Only a microwave having a prescribed frequency out of the microwaves returned by a capacitance between the gate electrode 6 and the drain electrode 8 of a FET9, resonates at a serial resonance circuit consisting of a dielectric resonator 100, and a capacitor 14, and it is reflected to a gate electrode 6 side, therefore, it oscillates at the frequency. It is possible to constitute the device in a very small shape because the coaxial dielectric resonator 100 is formed through the use of the conductor films 101 and 102, and is operated in a TEM mode. Furthermore, it is possible to change an oscillation frequency by changing capacitance between the internal conductor 101, and the conductor bar 17 by the inserting length of the conductor bar 17 to a cylindrical insulator 18. A temperature characteristic can be improved by constituting a conductor plate 16 and the conductor bar 17 of a metal with a small linear coefficient of expansion.