A method of manufacturing a semiconductor device includes forming a plurality of dummy line patterns arranged at a first pitch on an underlying region, forming first mask patterns having predetermined mask portions formed on long sides of the dummy line patterns, each of the first mask patterns having a closed-loop shape and surrounding each of the dummy line patterns, removing the dummy line patterns, forming a second mask pattern having a first pattern portion which covers end portions of the first mask patterns and inter-end portions each located between adjacent ones of the end portions, etching the underlying region using the first mask patterns and the second mask pattern as a mask to form trenches each located between adjacent ones of the predetermined mask portions, and filling the trenches with a predetermined material.