An RF circuit component according to the present invention includes a waveguide 1 and at least one resonator 2, which is arranged inside the waveguide 1. The resonator 2 includes at least one patterned conductor layer, which is parallel to a plane that crosses an H plane, and resonates at a lower frequency than a cutoff frequency, which is defined by the internal dielectric constant, shape and size of the waveguide 1, thereby letting an electromagnetic wave, having a lower frequency than the cutoff frequency, pass through the inside of the waveguide 1.