At least one surface of a plate member made of ZnSe has a concave-and-convex structure in which a projecting section and a groove section are formed at a spatial cycle equal to or lower than the wavelength of carbon dioxide laser light to thereby provide a substrate body. On a surface of the concave-and-convex structure, an antireflection film is layered that has a lower refractive index than that of ZnSe to carbon dioxide laser light. By this configuration, the polarization state of transmitted carbon dioxide laser light is converted from a linear polarization to a circular polarization or the like.