There is provided a composite thin film-holding substrate in which a composite thin film (4) comprising a filler (2) having a refractive index lower than that of a substrate (1) and a binder (3) having a refractive index higher than that of the filler (2) is formed on a surface of the substrate (1). Light is efficiently scattered when passing through the composite thin film (3) which comprises the filler (2) and the binder (3) having different refractive indexes from each other. In addition, the refractive index of a composite thin film (4) comprising a filler (2) having a low refractive index is low. As a result, the discharge efficiency of light which passes through the composite thin film (4) from the substrate (1) to the external is improved.