To relax the field concentration on the ends of field plate electrodes, without comparatively increasing the thickness of an insulation layer by partly burying insulator plates between a first and second insulation layers beneath the ends of the field plate electrodes.

Non-doped polycrystalline Si-made insulator plates 8 are buried between a first and second insulation layers 5, 6 so as to raise the ends of field plate electrodes 7. This allows the thicknesses d1, d2 of the insulation layers between the field plate electrodes 7 and semiconductor substrate 1 to be increased away from p-n junctions 3 to outside diffused regions 2. If hence a depletion layer 10 is formed around the p-n junctions 3 to result in generation of an electric field, the field concentration at the ends of the field plate electrodes 7 can be relaxed to improve the breakdown field intensities at the p-n junctions 3 and the insulation layer beneath the ends of the field plate electrodes 7.