To provide a variable capacitor having a large rate of change in dielectric constant with an electric field application and low dielectric loss in a microwave/millimeter wave band.

A variable capacitor is a solid solution having a composition of a general formula of (A1_xSr_1-x)(B1_xTi_1-x)O₃ (in which, A1 is at least one kind of element selected from the group consisting of Ca, Sr, Bi and Pb, and B1 is at least one kind of element selected from the group consisting of Ti, Fe and Zr), where one end composition substance is SrTiO₃ as a quantum paraelectric with a perovskite structure; the other composition substance is a complex oxide which is all-rate solid soluble to the SrTiO₃ as the above end composition substance and has a perovskite structure with the Curie temperature Tc higher than room temperature; and a ferroelectric layer 3 is equipped which is formed of a ferroelectric material with composition ratio x selected so that the Curie temperature Tc of the above solid solution is room temperature or less.