When a wiring structure is formed by a trench-first dual damascene method, a first hard mask for forming via holes and a second hard mask for forming wiring trenches are sequentially formed on an interlayer insulating film, openings are formed at the first hard mask while using the second hard mask as a mask, and thereafter, the openings are expanded in a lateral direction by an isotropic etching to form openings, via holes are formed by etching the interlayer insulating film while using the first hard mask and the second hard mask as masks, and wiring trenches communicating with the via holes are formed by etching the interlayer insulating film while using the second hard mask as a mask.