Four kinds of dielectrics in the form of a mixture of ZrO.sub.2 and TiO.sub.2 having a refractive index of 2.07, MgF.sub.2 having a refractive index of 1.37, Al.sub.2 O.sub.3 having a refractive index of 1.61, and TiO.sub.2 having a refractive index of 2.27 are laminated on a transparent glass substrate in twenty layers. Two kinds of unpolarized light beam splitters having ratios of the reflected light to the transmitted light of 7:3 and 8:2, respectively, are thereby obtained. Both kinds of unpolarized light beam splitters have the twentieth layer, that is the outermost layer, formed with MgF.sub.2. The difference between the reflectance of the polarizing P component and the reflectance of the polarizing S component of the reflected light is set to be below 10%.