Abstract: An optical multilayer-film filter formed by laminating dielectric thin films on a transparent substrate has a structure in which a dielectric multilayer film formed by alternately laminating high-refractive-index material layers and low-refractive—index material layers is formed on one surface of the transparent substrate, a dielectric monolayer film is formed on the other surface of the transparent substrate, and the dielectric monolayer film is composed of a dielectric material having substantially the same refractive index as that of the transparent substrate. With this structure, a warp width of the substrate due to stresses of the dielectric thin films laminated in the transparent substrate can be further reduced, thereby achieving an optical multilayer-film filter which is protected from optical strain.