PURPOSE: To form a dielectric compound film excellent in breakdown life even in the case of the dielectric compound film containing a silicon nitride film, by a method wherein the silicon nitride film is formed by low pressure CVD method using SiH2Cl2 gas and NH3 gas wherein the substratum heating temperature is kept lower than or equal to 750°C.

CONSTITUTION: Firstly, as a substratum, a plurality of P-type silicon substrates 31 whose resistivity is, e.g. 3-5Ωcm are prepared. Next, each of the silicon substrates 31 is arranged in an O2 gas atmosphere diluted by N2 gas at a temperature of, e.g. 900°C for about 30 minutes, and the surface is oxidized, thereby forming a silicon oxide film 33 of 60 in thickness on each silicon substrate 31. On the silicon oxide film 33 on the silicon substrates 31, a silicon nitride film 35 of 100 in thickness is formed by reduced pressure CVD method using SiH2Cl2 gas and NH3 gas wherein the heating temperature of the substratum is lower than or equal to 750°C. Thereby the local fluctuation of film thickness can be reduced, and the electric field can be prevented from locally concentrating, so that breakdown life can be improved.