Abstract: A semiconductor device having a passive element whose characteristic is adjustable even after manufacture by applying back bias voltage is provided. Formed on a main surface of a SOI substrate comprising a supporting substrate, a BOX layer, and an SOI layer is a MOS varactor comprising a gate dielectric formed on a surface of the SOI layer, a gate electrode formed on the gate dielectric, and a n+ type semiconductor region formed in the SOI layer located on both sides of the gate electrode. The MOS varactor, is configured so that capacitance formed by the SOI layer, gate dielectric, and gate electrode is varied by applying bias voltage to the supporting substrate (p type well) under the gate electrode.