PURPOSE: To obtain a semi-transparent mirror having a flat spectral characteristic and a wide band by forming it with a structure which has laminated alternately a high refractive index dielectric layer and a low refractive index dielectric layer by several layers on a transparent substrate, and setting an optical film thickness of those layers to a prescribed thickness, respectively.

CONSTITUTION: For instance, a semi-transparent mirror of a five layer structure is formed by laminating alternately low refractive index dielectric layers L22, 24 and high refractive index dielectric layers H21, 23 and 25 on a transparent substrate G. By counting from an air side, the first layer 25 is a high refractive index dielectric layer. As for an optical film thickness of each layer, the layers 23W25 of the upper side are λ/4, and two layers 21, 22 of the lower side are λ/2. In this regard, λ is a design reference wavelength, and set to a range of a visible area of 400W700nm. As for a material of the high refractive index dielectric, TiO2, ZrO2 or their mixture are used, and as for a material of the low refractive index dielectric, MgF2, SiO2, etc. are used.