A plurality of one-quarter wavelength coplanar resonators 5a to 5d are formed in series on a dielectric substrate 1, and coplanar input/output terminal sections 4a and 4b are formed on the dielectric substrate at opposite ends of the series connection for coupling with resonators 5a and 5d, respectively. A center conductor line width w.sub.1 of each of the resonators 5a to 5d is equal to a center conductor line width w.sub.io of each of the input/output terminal section 4a and 4b, but a ground conductor spacing d.sub.1 of each of the resonators 5a to 5d is greater than a ground conductor spacing d.sub.io of each of input/output terminal section 4a and 4b. Maintaining the accuracy of design is facilitated and a reduction in the maximum current density in the resonator is enabled.