PURPOSE: To improve the effect of preventing the reflection of IR rays by laminating many layers of specific layers on a Ge substrate.

CONSTITUTION: A ZnS film or ZnSe film having an intermediate refractive index, a Ge film having a high refractive index, the above-mentioned film having the intermediate refractive index, and a film of any among BaF2, LaF2, CaF2 and SrF2 having a low refractive index are provided on a Ge substrate successively from the substrate side. The respective thicknesses of the 1st W the 4th layers designated as d1, d2, d3, d4 and the respective refractive indices designated as n1, n2, n3, n4 are preferably set at n1d1:n2d2:n3d3:n4d4= 1:2:5.4:4.4.