A method of fabricating a semiconductor device includes forming trench-like recesses in a semiconductor substrate, the recesses including one or more recesses each of which has an opening width of not more than a predetermined value, forming a first insulating film above the substrate after the recesses have been formed, so that one or a plurality of voids are formed in the one or more recesses whose opening widths are not more than the predetermined value, removing part of the first insulating film so that a beam is left which spans the openings so that the beam passes over upper surfaces of the one or more recesses and so that at least the voids are exposed in a portion of the substrate except the beam, and filling the voids in the recesses with a material with fluidity, thereby forming second insulating films in the recesses.