To provide a highly reliable semiconductor device and a method for manufacturing the semiconductor device, where defects such as a short between a gate electrode layer and a semiconductor layer and a leakage current, which would otherwise be caused due to a coverage defect of the semiconductor layer with an insulating layer, can be prevented. In order to form a plurality of semiconductor elements over an insulating surface, a semiconductor layer is not separated into a plurality of island-shape semiconductor layers, but instead, element isolation regions, which electrically insulate a plurality of element regions functioning as semiconductor elements, are formed in one semiconductor layer, i.e., a first element isolation region with high resistance and a second element isolation region which has a contact with the element region and has a conductivity type opposite to that of the source and drain regions of the element region.