A semiconductor substrate has a sensor disposed in a surface layer on an entrance surface thereof for receiving incident light, and an intermediate-refractive-index film is disposed on the entire entrance surface of the semiconductor substrate either directly or with an insulating film interposed therebetween. The intermediate-refractive-index film has a refractive index lower than the semiconductor substrate and a low hydrogen permeability thin film is disposed on an entrance surface of the intermediate-refractive-index film and having refractive index lower than the intermediate-refractive-index film the thin film being permeable to hydrogen. The intermediate-refractive-index film serves as a reflection-resistant film. Reflections of incident light applied through the thin film and the reflection-resistant film of the sensor of the semiconductor substrate are suppressed. The intermediate-refractive-index film has a hole defined therein for passage of hydrogen therethrough upon hydrogen alloying. After the thin film is formed on the surface of the intermediate-refractive-index film, hydrogen passes through the hole in the intermediate-refractive-index film to reach the surface of the semiconductor substrate near the sensor upon hydrogen alloying.