Formed on a dielectric substrate 11 are a center conductor 12 having a length L1 which is equivalent in electrical length to one quarter wavelength and ground conductors 13 disposed on the opposite sides of the center conductor with a gap portion therebetween in coplanar manner. The center conductor 12 and the ground conductors 13 located on the opposite sides thereof are connected together by shorting ends 14 resulting in forming corner areas, respectively whereby obtaining a coplanar waveguide resonator, wherein the edge line of the shorting end 14 is recessed to have an arcuate curve configuration so that each corner area has an angle of greater than 90° to reduce power current concentration at the corner points in the respective corner areas.