PURPOSE: To simplify the circuit constitution and to facilitate the manufacture with more simplified structure by providing a resonance frequency adjusting means varying the parallel capacitance to a parallel resonance circuit equivalently through the increase/decrease in the inserted length.

CONSTITUTION: A cylindrical hole 2 having a proper diameter is penetrated to the center of a pillar dielectric block 1 having a length nearly equal to a 1/4 wavelength and a conductor film is coated to the entire outer circumferential face of side and bottom faces 3, 8 and the inner circumferential face 4 of the cylindrical hole 2 except the upper face 5 of the dielectric block 1 to constitute a 1/4 wavelength coaxial resonator. Then a small hole 6 having far small diameter and length than those of the cylindrical hole 1 is made to the dielectric block 1 at a proper position apart from the cylindrical hole 2 at the center of the upper face 5, a conductor rod 7 penetrating the upper face 5 is connected to ground via a side face conductor film 3 is inserted into the small hole 6 and the length of the inserted part of the conductor rod 7 into the dielectric block 1 is increased/decreased and fixed to a desired position. Thus, the entire constitution is simplified.