A semiconductor device includes a pair of selective gate lines formed above a semiconductor substrate, plural word lines formed above the substrate, plural contact plugs located between the selective gate lines, a first insulator formed in the trenches between the word lines, the first insulator including a first insulating film having a first upper surface flush with the substrate surface, a second insulator formed in the trenches between the contact plugs and including second and third insulating films, and a boro-phosphor-silicate glass film formed on the third insulating film and between the contact plugs. The second insulating film is of a kind same as the first insulating film. The third insulating film has a higher resistance to a wet etching process than the second insulating film. An interface between the second and third insulating films is located between a bottom and an upper end of the trench.