A dielectric filter includes a case (15), a substrate (6) having microstrip lines formed thereon, and a dielectric plate (3) having non-electrode parts (4a, 4b, 4c) serving as dielectric resonators. The case (15) includes a supporting part for supporting the lower surface of the dielectric plate (3) and includes a side wall surrounding the side faces of the dielectric plate (3) wherein the supporting part and the side wall are formed in an integral fashion. The substrate (6) is bonded to the case (15) and the dielectric plate (3) is mounted on the supporting part of the case. A metal cover (8) is then placed on the case (15) such that the opening of the case is closed with the cover.
In the dielectric filter constructed in the above-described manner, warping of the case (15) for supporting the dielectric plate (3) is suppressed and thus the stress exerted on the dielectric plate (3) is reduced. As a result, the dielectric plate (3) is prevented from being separated from the case (15) and also prevented from having a crack. The above-described structure also allows the dielectric filter to be formed into a small size. The invention also provides a duplexer and a communication device using such a dielectric filter.