A composite substrate includes a single crystal support substrate containing first element as a main component; an oxide single crystal layer provided on the single crystal support substrate and containing a second element (excluding oxygen) as a main component; and an amorphous layer provided in between the single crystal support substrate and the oxide single crystal layer and containing a first element, a second element, and Ar, the amorphous layer having a first amorphous region in which proportion of the first element is higher than proportion of the second element, and a second amorphous region in which the proportion of the second element is higher than the proportion of the first element, concentration of Ar contained in the first amorphous region being higher than concentration of Ar contained in the second amorphous region and being 3 atom % or more.