In order to form a plurality of semiconductor elements over an insulating surface, in one continuous semiconductor layer, an element region serving as a semiconductor element and an element isolation region having a function to electrically isolate element regions from each other by repetition of PN junctions. The element isolation region is formed by selective addition of an impurity element of at least one or more kinds of oxygen, nitrogen, and carbon and an impurity element that imparts an opposite conductivity type to that of the adjacent element region in order to electrically isolate elements from each other in one continuous semiconductor layer.