A semiconductor device includes a plate of semiconductor layer, an insulator layer formed on the plate of semiconductor layer and brought into contact with the plate of semiconductor layer by at least two adjacent faces, a thickness of the insulator layer in the vicinity of a boundary line between the two adjacent faces being larger than that of the insulator layer in a region other than the vicinity of the boundary line, and a band of conductor layer formed facing a middle portion of the plate-like semiconductor layer via the insulator layer.