Abstract: A semiconductor device comprising an element separation insulator layer (5, 6, 29) is formed to surround an active region and insulate this active region from other active regions, and to have substantially uniform height throughout its periphery on a main surface of a semiconductor substrate (1). A semiconductor layer (14) is formed flat on an entire area of the active region surrounded by this element separation insulator layer (5, 6, 29) and essentially coplanar to it. The surface of the semiconductor layer (14) is used as an element formation region. This semiconductor device can remarkably reduce an error or difference between design size of the element pattern and actual size of the element, and also can prevent a short-circuiting between conductive wirings by debris.