A method for manufacturing a semiconductor device is disclosed. The method includes forming a shallow trench isolation (STI) region extending in a first direction on a semiconductor substrate, forming a mask layer extending in a second direction that intersects with the first direction on the semiconductor substrate and forming a trench on the semiconductor substrate by using the STI region and the mask layer as masks. In addition, the method includes forming a charge storage layer so as to cover the trench and forming a conductive layer on side surfaces of the trench and the mask layer. Word lines are formed from the conductive layer on side surfaces of the trench that oppose in the first direction by etching. The word lines are separated from each other and extend in the second direction.