A filter is provided which maintains a low insertion loss characteristic of a filter contained in a casing with a very simple construction that the internal wall of the casing is formed by a superconductor. A coplanar waveguide filter 22 comprises a dielectric substrate 1, a plurality of resonators 5a, 5b, 5c and 5d and input/output terminal sections 4a and 4b, each of which is formed by a center conductor 2 and ground conductors 3a and 3b, both formed on the same surface of the dielectric substrate 1, with the ground conductors 3a to 3d being formed on the opposite sides of and in parallel relationship with the center conductor 2. The filter 22 is contained within a casing 21 having an internal wall, the surface of which is formed with a layer of superconductor 23. By way of example, a high temperature superconductor such as lanthanum-, yttrium-, bismuth- or thallium-superconductor is deposited as a film on a substrate of a metal oxide material such as MgO, SrTiO.sub.3, LaGaO.sub.3, LaAlO.sub.3 to provide a superconductor filmed substrate 25, which is applied to the internal surface of the casing 21. Electromagnetic power which is irradiated from the filter 22 does not produce a power loss when it impinges on the layer of superconductor 23 in its superconducting state, but is reflected therefrom to be absorbed by the filter 22, thus reducing the filter insertion loss.