According to one embodiment, a method of fabricating a semiconductor device, including, selectively forming a first film as a core member on a film to be processed, forming a second film on a side surface and an upper surface of the core member, and on an upper surface of the film to be processed to cover the film, the second film which is constituted with same material as the first film and is doped with impurities being different in amount from impurities in the first film, removing the second film on the core member and on the film to be processed to form a sidewall mask constituted with the second film on the side surface of the core member, selectively removing the core member, and etching the film to be processed using the sidewall mask film as a mask.