PURPOSE: To enable the titled device which has attained the improvement in gate withstand voltage and in integration to be obtained with high yield by providing an element region whose bottom is a smooth inclined surface.

CONSTITUTION: An insulation film 23 is formed on the semiconductor layer 22 of a substrate 21 produced by lamination of an Si semiconductor layer 21 on a sapphire substrate 20. The region except that immediately under the insulation film 23 is etched in such a manner that the semiconductor layer 21 remains on the sapphire substrate 20. An insulation film 24 is deposited on the remaining insulation film 23 and semiconductor layer 21. An element region 27 is obtained on the sapphire substrate 20 by removing the insulation films 23 and 24. Next, a thin gate oxide film 28 is formed on the surface of the element region 27. The bottom of the region 27 is a smooth inclined surface 26, and the improvement in gate withstand voltage and the titled device of high reliability can be obtained with high yield by uniform formation of the gate oxide film 28.