This invention decreases deformation of a plastic substrate caused by thermal expansion and contraction in an evaporation process of forming a multi-layered film of an ND filter, thus helping prevent wrinkles. The plastic substrate serving as the base of the ND filter is made of a polyimide-based plastic material having a glass transition temperature of 200.degree. C. or more, has a total transmittance of light of 90% or more, and has a haze factor of 0.5% or less. As the multi-layered film, Al.sub.2O.sub.3 films for reduction of the reflectance and TiO.sub.x films for reduction of the transmittance are alternately formed on the substrate.