A method for fabricating a semiconductor device includes: forming a stack body including first sacrificial layer structures, preliminary horizontal layers, and second sacrificial layer structures over a lower structure; forming a main hard mask layer over the stack body; forming a mesh-shaped hard mask pattern over the main hard mask layer; forming a main hard mask pattern by etching the main hard mask layer using the mesh-shaped hard mask pattern as an etch barrier; forming a plurality of isolation openings by etching the stack body using the main hard mask pattern as an etch barrier; and forming a plurality of isolation layers that fill the isolation openings.