According to one embodiment, a method of fabricating a semiconductor device is disclosed. The method includes the steps of: forming a tunnel insulating film on a semiconductor substrate; forming a floating gate electrode on the tunnel insulating film; and forming a silicon nitride film including a low-density silicon nitride film and a high-density silicon nitride film on the floating gate electrode. The method also includes the steps of: forming an isolation trench thereby to expose the low-density silicon nitride film exposed at least in a portion of a side surface of the isolation trench; forming an isolating insulating film covering an internal surface of the isolation trench; removing the silicon nitride film; and forming an interelectrode insulating film and a control gate electrode both covering the floating gate electrode and the isolating insulating film.