A thermally oxidized heterogeneous composite substrate provided with a single crystal silicon film on a handle substrate, said heterogeneous composite substrate being obtained by, prior to a thermal oxidization treatment at a temperature exceeding 850° C., conducting an intermediate heat treatment at 650-850° C. and then conducting the thermal oxidization treatment at a temperature exceeding 850° C. According to the present invention, a thermally oxidized heterogeneous composite substrate with a reduced number of defects after thermal oxidization can be obtained.