<p>Disclosed is an SOI substrate comprising a supporting substrate composed of a semiconductor single crystal and an active layer composed of a semiconductor single crystal which is bonded onto the supporting substrate via an oxide film. By forming the oxide film only on the active layer and etching the surface of the active layer selectively using reactive radicals produced by plasma etching, the active layer is formed to have a thickness of 10-200 nm and the thickness difference in the entire active layer is set not more than 1.5 nm.</p>
