PURPOSE: To control an antiresonance point by using a trap which uses a dielectric coaxial resonator.

CONSTITUTION: A transmission line 6 having an open tip is composed of, for example, a semirigid cable, and both terminals of the series circuit of the dielectric coaxial resonator 1 and a capacitor 2 are connected to the center conductor 4 and jacket conductor 5 of the cable 6 respectively. The cable 6 features inductively or capacitively according to its length, so the length is so set that antiresonance is caused at a specific frequency with the combination of a resonator 1, the capacitor 2, and the cable 6. The trap using this resonator 1 controls the antiresonance point.