The HVIC includes a dielectric layer and an SOI active layer stacked on a silicon substrate, a transistor formed in the surface of the SOI active layer, and a trench isolation region formed around the transistor. The dielectric layer includes a first buried oxide film formed in the surface of the silicon substrate, a shield layer formed below the first buried oxide film opposite the element area, a second buried oxide film formed around the shield layer, and a third buried oxide film formed below the shield layer and the second buried oxide film. Therefore, the potential distribution curves PC within the dielectric layer are low in density and a high withstand voltage is achieved.