To lower a process temperature for an SOQ substrate manufacturing process to reduce the degree of surface roughness of an SOQ film and provide a high-quality SOQ substrate.Hydrogen ions are implanted to a surface of a single crystal Si substrate 10 through an oxide film 11 to uniformly form an ion implanted layer 12 at a predetermined depth (average ion implantation depth L) from the surface of the single crystal Si substrate 10, and a bonding surface of the substrate undergoes a plasma treatment or an ozone treatment. An external shock is applied onto the single crystal Si substrate 10 and quartz substrate 20, which are bonded together, to mechanically delaminate a silicon film 13 from a single crystal silicon bulk 14. In this way, the SOQ film 13 is formed on the quartz substrate 20 through the oxide film 11. To further smooth the SOQ film surface, hydrogen heat treatment is performed at a temperature of 1000° C. or less below a quartz glass transition point. When measuring surface roughness of an SOQ film after performing hydrogen heat treatment on a sample having surface roughness of about 5 nm in terms of RMS average value immediately after delamination, a satisfactory measurement result of 0.3 nm or less in terms of RMS average value was obtained.