PURPOSE: To contrive to improve the characteristics of the low withstand voltage part leaving the characteristics of the high withstand voltage part as they are held by a method wherein a process, wherein an impurity of the same conducting type as that of the substrate is diffused in the shallow single crystal island region to be used as the low withstand voltage part, is added.

CONSTITUTION: The oxide film on the bottom surface of a recessed part 4, which is used as the low withstand voltage part, is removed by a photolitho etching, and after that, an N-type impurity is diffused to the part to become the surface of the island in the low withstand voltage part, and a diffusion region 6 having the desired impurity concentration is formed. An oxide film 5 is removed, a thermal oxide film 7 is formed, and after a photolitho- etching and an anisotropic etching, V-shaped grooves 8 are provided and the island region, which is used as the low withstand voltage part, is formed. Then, an oxide film 9, by which each island region is insulatingly isolated, is formed, subsequently polycrystalline silicon a layer 10, which is used as the supporting matter, is deposited. After that, the main surface on the side of a polycrystalline silicon substrate 1 is polished until the points of the V-shaped grooves 8, 8 are made to expose and the semiconductor substrate having a low withstand voltage single crystal island 11 and a high withstand voltage single crystal island 12, which are respectively isolated into a dielectric and are different in depth, is obtained.