To provide a practical SOS substrate with few crystal defects and sufficient surface roughness.

A sapphire single crystal substrate 100 is prepared and a first silicon epitaxial film deposited by an ordinary pressure CVD method is formed on a first main face 102 of the substrate. A second silicon epitaxial film deposited by a low-pressure CVD method is formed on the film. Si+ions are implanted from a surface of the second silicon epitaxial film. The first and second silicon epitaxial films are made into an amorphous state, and an annealing processing is performed in a hydrogen atmosphere. Thus, solid phase epitaxial regrowth is performed. The second silicon epitaxial film 106" which is solid-phase-epitaxially regrown is oxidized until it changes to a silicon oxide film 110 by oxidizing it by thermal oxidation. The silicon oxide film is etching-removed, and a SOS substrate 120 consisting only of the first silicon epitaxial film 104" which is solid-phase-epitaxially regrown is obtained.