Electrodes (2) and (3) are formed on the front face (1A) and the rear face (1B) of a dielectric substrate (1). Fan-shaped apertures (4A) and (4B) forming a resonator (4) are formed in the electrodes (2) and (3) such that the fan-shaped aperture (4A) opposes the fan-shaped aperture (4B). Accordingly, two parameters, that is, the radius and the central angle, of the fan-shaped apertures (4A) and (4B) can be used to set the resonant frequency of the resonator (4), thus improving the flexibility in design of the resonator (4).