In an optical filter, a thin film is formed by laminating low refractive index layers alternatingly from a substrate side with high refractive index layers. The thin film is provided with first, second, and third laminated portions. In the first laminated portion, the refractive indices of the high refractive index layers become gradually higher. In the second laminated portion, the refractive indices of the high refractive index layers are substantially equal to the highest refractive index of the high refractive index layers constituting the first laminated portion. In the third laminated portion, the refractive indices of the high refractive index layers become gradually lower, and the refractive indices of the low refractive index layers are substantially equal to the lowest refractive index of the low refractive index layers constituting the second laminated portion. The low refractive index layers of the first and second laminated portions have a low refractive index layer uniform portion or a low refractive index layer decreasing portion.