COMPOSITE SUBSTRATE

Provided is a composite substrate having a semiconductor layer wherein diffusion of a metal is suppressed. This composite substrate has: a single crystal supporting substrate composed of an insulating oxide; a semiconductor layer, which has one main surface overlapping the supporting substrate, and which is composed of a single crystal; and a polycrystalline or amorphous intermediate layer, which is positioned between the supporting substrate and the semiconductor layer, and which has, as a main component, an element constituting the supporting substrate or an element constituting the semiconductor layer, and in which the ratio of accessory components other than the main component is less than 1 mass %.