To provide a varactor that prevents a Q value from lowering caused by voltage application in a desired frequency, maintains the high Q value regardless of the voltage application, and lowers an insertion loss and phase characteristics.

This varactor is manufactured by sequentially laminating on the support substrate 1 a lower conductive layer 2, a dielectric layer 3 whose capacity varies depending on the voltage application, and an upper conductive layer 6, and is formed with a slope formed at a capacity formation part at least in either of the lower conductive layer 2 and the upper conductive layer 6 overlapping the dielectric layer 3 in a plan view, so that an acoustic wave generated in the dielectric layer can be reflected to the outside of the dielectric layer. Compared with the Q value before the voltage application, the Q value after the voltage application is prevented from being increased greatly in the frequency range of a varactor so that the varactor employing a dielectric thin film of high permittivity can obtain the high Q value in a wide range of a frequency regardless of the voltage application.