The present fabrication method includes the steps of: providing a nitride film in a main surface of a semiconductor substrate; providing an upper trench, with the nitride film used as a mask; filling the upper trench with an oxide film introduced therein; removing the oxide film to expose at least a portion of a bottom of the upper trench and allowing a remainder of the oxide film to serve as a sidewall; providing a lower trench in a bottom of the upper trench, with the sidewall used as a mask; and with the upper trench having the sidewall remaining therein, providing an oxide film in the upper trench and the lower trench. This can provide a semiconductor device fabrication method and a semiconductor device preventing a contact from penetrating the device in an interconnection process.