To provide an optical thin film structure capable of efficiently dissipating heat in an optical thin film which is generated upon irradiating a surface of an X-ray mirror made up of the optical thin film with an X-ray. The optical thin film having an isotope purity higher than a natural isotope abundance ratio is formed on a mirror to increase heat conductivity of the optical thin film itself and quickly dissipate heat accumulated in the thin film to the outside of an optical system. Consequently, the mirror having high reflectively can be obtained, in which a fine structure of the optical thin film is by no means broken.