A semiconductor device includes a plurality of floating regions, an insulating layer and a capacitance forming portion. The plurality of floating regions are arranged on a surface of a semiconductor substrate in a row, wherein the plurality of floating regions are provided with insulating regions therebetween. The plurality of floating regions include a first floating region and a second floating region. The second floating region is located farther than the first floating region from an island region of a predetermined potential on the semiconductor substrate. The insulating layer is interposed between each of the plurality of floating regions and a semiconductor material layer of the semiconductor substrate. The capacitance forming portion forms an external capacitance in parallel with the capacitance of the insulating region between the first floating region and the island region of the predetermined potential.