When forming a silicon nitride film to protect and insulate a surface on which a silicon substrate has been ground or polishing, by use of a mixed gas containing SiH.sub.4, N.sub.2, and NH.sub.3 as a reaction gas, a film is formed by a single-frequency parallel-plate plasma CVD method. Thereby, even when the film forming temperature is made not more than an allowable temperature limit of an adhesive to adhere a support (for example, approximately 100.degree. C. or less, which is an allowable temperature limit when the adhesive is an ultraviolet curing resin), a high-quality film without exfoliation in a CMP step of the following step and with less leakage can be formed. This high-quality film is, if being prescribed by a refractive index, a film whose refractive index with respect to a wavelength of 633 nm is approximately 1.8 through 1.9.