PURPOSE: To realize the separation of a flat oxide film wherein bird's beak is not present, wiring capacitance can be reduced, and distortion and dislocation due to thermal stress are not caused by a method wherein, after a trench is formed in an Si substrate, an oxide film is buried while polysilicon or the like is left in the sidewall of the trench.

CONSTITUTION: Semiconductor substrates 1, 5 are selectively etched; polysilicon 7 or spin on glass is laminated on the whole surface; then by anisotropic etching, only the sidewall 8 of a region to be etched of the substrates 1, 5 is left; after a silicon oxide film 10 is laminated on the whole surface, the silicon oxide film 10 is buried only in the region to be etched of the substrates 1, 5 by etch- back method. For example, after an N-type silicon epitaxial layer 5 is formed on an N+ silicon substrate 1, an insulating film 3 is formed on the surface; photo resist 4 is selectively left, and the insulating film 3 and the Si substrate 5 are continuously etched; the resist 4 is eliminated; the surface of a trench part 2 is oxidized; a thin silicon oxide film 6 is formed: then polysilicon is grown so as to have a thickness of (t).