A semiconductor device includes: a silicon substrate with semiconductor elements; an isolation trench formed in the silicon substrate for isolating active regions in the silicon substrate, the isolation trench having a trapezoidal cross sectional shape having a width gradually narrowing with a depth from the surface of the silicon substrate; a first liner insulating film formed on the surface of the trench and made of a silicon oxide film or a silicon oxynitride film having a thickness of 1 to 5 nm; a second liner insulating film formed on the first liner insulating film and made of a silicon nitride film having a thickness of 2 to 8 nm; and an isolation region burying the trench defined by the second liner insulating film.