A composite substrate comprising a monocrystalline support substrate made of an insulating material and a monocrystalline semiconductor part disposed as a layer on the upper surface of the support substrate. An interface region having a thickness of 5 nm from the bonding interface between the support substrate and the semiconductor part towards the semiconductor part side includes a metal comprising: a metal element excluding the materials constituting the main components of the support substrate and the semiconductor part; and an inert element selected from the group consisting of Ar, Ne, Xe, and Kr. The number of atoms per unit area of the inert element is greater than that of the metal and smaller than that of the element constituting the semiconductor part.