A semiconductor device includes a P-type low potential region, an N-type first region, an N-type second region, an N-type third region, an annular trench, and a P-type isolation region. The N-type first region is provided on the principal surface of a P-type SOI layer provided to a P-type SOI substrate. The N-type first region has a concave portion. The N-type third region is provided inside the concave portion of the N-type first region so as to be away from the edge of the concave portion. A level-shift device is formed on the surface of the N-type third region. The P-type isolation region is a slit region extending in U-shape along the boundary between the N-type third region and the concave portion of the N-type first region.