PURPOSE: To obtain a high reflectivity over a wide range by selecting antimony trioxide or pentaoxide as a material having the high complex index of refractive to X-rays of a 20 to 44 wavelength region, of two kinds of the materials which are different in the complex index of refractive.

CONSTITUTION: Two kinds of the materials which are different in the complex index of refraction of the materials in the X-ray region are laminated alternately at adequate thicknesses to form multilayered films. The two materials which increases the difference in the complex index of refraction are effective. The complex index of refraction of the materials in the above-mentioned X-ray region is given by prescribed equation. The coefft. of absorption is decreased by using Sb2O3 and Sb2O5 which are the oxides of Sb in place of the Sb. Further, these materials have the tendency to increase the refractive index by Sb2. The Sb2O3 and Sb2O5 are, therefore, usable as the materials having the high refractive index over the wavelength region of a 'window of water', of two kinds of the materials constituting the multilayered reflecting films for X-rays. Particularly, these materials are useful at 27 to 44 on the long wavelength side in the 'window of water'. Of two kinds of the materials constituting the multilayered films, nickel mononitride, copper and cobalt are more preferably seleced as the low refractive index material.