A semiconductor device enabling word lines to be arranged at close intervals, comprising a plurality of memory transistors arranged in an array and a plurality of word lines serving also as gate electrodes of memory transistors in a same row, extending in a row direction, and repeating in a column direction, where insulating films are formed between the plurality of word lines to insulate and isolate the word lines from each other and where a dimension of separation of word lines is defined by the thickness of the insulating films.