A center conductor having a length L1, which is equivalent in electrical length to one quarter wavelength, and ground conductors disposed on the opposite sides of the center conductor with a gap portion therebetween in coplanar manner are formed on a dielectric substrate. The center conductor and the ground conductors located on the opposite sides thereof, and are connected together by shorting ends. This results in the formation of corner areas, respectively, whereby obtaining a coplanar waveguide resonator. An edge line of the shorting end is recessed to have a curve configuration so that each corner area has an angle greater than 90 degrees, which reduces power current concentration at the corner points in the respective corner areas.