PURPOSE: To attain stability against mechanical vibration and frequency characteristic by forming an end of a frequency adjusting rod not opposite to a dielectric resonator as a cylinder having a vertically split solt and inserting a wedge to the cylinder inner face so as to fix the frequency adjusting rod to a case.

CONSTITUTION: A mounting part 13 is formed as a cylindrical projection at the outer circumference of a through-hole of the case 1, a screw thread is engraved on the inner wall of the projection and screwed with a screw thread engraved on the outer circumference of the frequency adjusting rod 9. The end of the frequency adjusting rod 9 at the cavity side is provided with an adjusting disc 9b made of a metallic disc and the other end is formed as a cylinder 9d having a vertically split slot 9c. The wedge 10 consists of a conical part 10a and a larger part 10b and the outer diameter of the root of the conical part 10a is selected larger than the inner diameter of the cylinder 9I. In inserting the wedge 10 until the bottom face of the larger part 10b is in contact with the frequency adjusting rod 9, the conical part 10a pushes and spreads the inner wall of the cylinder 9d of the frequency adjusting rod 9 and the frequency adjusting rod 9 is fixed tightly to the mounting part 13 of the case 1.