A dielectric isolation type semiconductor device and a manufacturing method therefor achieve high dielectric resistance while preventing the dielectric strength of the semiconductor device from being limited depending on the thickness of a dielectric layer and the thickness of a first semiconductor layer. A semiconductor substrate (1) and an n− type semiconductor layer (2) are bonded to each other through a buried oxide film layer (3). A first porous oxide film area (10) is formed in the semiconductor substrate in a state contacting with the buried oxide film layer. A power device is formed on the n− type semiconductor layer. The first porous oxide film area is formed in an area including a location right under a first main electrode (6) and extending from the first main electrode side up to a range of more than 40% of a distance (L) between the first and second main electrodes (6, 7).