The present invention relates to a semiconductor integrated circuit device of high degree of integration. A first element region and a second element region are provided with a field insulating film interposed therebetween on a semiconductor substrate of one conductivity type. Impurity regions of one conductivity type having a high impurity concentration are separately formed in the substrate at locations of the first and second element regions, respectively. The respective impurity regions are wider that the respective element regions, and extends under end portions of the field insulating film but not under the center portion thereof. A wiring layer is provided on the center portion of the field insulating film beneath which no impurity region exists. The element regions are isolated from each other by a predetermined threshold voltage determined by the end portions of the field insulating film and by the underlying high impurity regions. Further, parasitic capacity can be reduced between the wiring layer and the semiconductor substrate since no impurity region having high concentration exists under the wiring layer.