The development of phase mask using the refractive index variation due to the UV-photodecomposition of the polysilane was examined. It was proven that the refractive index due to the UV-photodecomposition of the poly[methyl(phenyl)silane] changed from 1.70 to 1.56 by means of the ellipsometry measurements. Furthermore, it was confirmed that information of the phase difference could be recorded by providing the pattern of the UV photodecomposition of the polysilane.