In a semiconductor device, a gate electrode, an impurity diffused region, a body potential fixing region, a first insulator, and a dummy gate electrode are provided on top of an SOI substrate consisting of an underlying silicon substrate, a buried insulator, and a semiconductor layer. The impurity diffused region is a region formed by implanting an impurity of a first conductivity type into the semiconductor layer around the gate electrode. The body potential fixing region is a region provided in the direction of an extension line of the length of the gate electrode and implanted with an impurity of a second conductivity type. The first insulator is formed at least in the portion between the body potential fixing region and the gate electrode. The dummy gate electrode is provided on the first insulator between the body potential fixing region and the gate electrode.