An Integrated Circuit (IC) semiconductor device includes: field insulating layers buried in field trenches disposed apart from each other inside a substrate; active regions defined by the field insulating layers; and active fins disposed on the active regions and protruding from surfaces of the field insulating layers. The field insulating layers include a first subfield insulating layer and a second subfield insulating layer, and a surface of the first subfield insulating layer is disposed at a level lower than a level of a surface of the second subfield insulating layer.