A semiconductor device includes a pair of select gate structures which are opposed to each other and which are formed in a select transistor formation area, each of the select gate structures including a gate insulating film formed on a semiconductor substrate and a gate electrode formed on the gate insulating film, and a pair of memory cell gate structure groups which are formed in a pair of memory cell formation areas between which the select transistor formation area is interposed and each of which has a plurality of memory cell gate structures arranged at the same pitch, the pair of select gate structures having sides which are opposed to each other, and at least the upper portion of each of the opposed sides of the select gate structures being inclined.