PURPOSE: To obtain an IC of high speed and high density by reducing the variation of diffusion depths and dimensions due to oxidation by a method wherein an oxidation resistant insulation film is provided on a diffused layer and a poly Si.

CONSTITUTION: An insulation isolation film 2, an N+ layer 3, and a poly Si electrode 4 are provided on a P type Si substrate 1. Thereat, a Si3N4 film 7 is grown in vapor phase on a thermal oxide film 5 formed. Thereafter, the formation of a MOS IC as fixed can block the infiltration of O2 in the presence of the film 7; therefore, compared with the MOS IC without the film 7, the variation of the mounting dimensions of a diffused layer and a poly Si can be reduced, resulting in high integration and high speed, even through the high temperature treatment in O2.