A sample, in which a dielectric film having a dielectric constant equal to or larger than 50 (based on electrical measurement) is formed on a substrate, is measured by an ellipsometer while a model corresponding to the sample is formed based on effective medium approximation (EMA). A film corresponding to the dielectric film of the model includes void volume fraction between 60% and 90%. A calculated value based on the model is compared with a value measured by the ellipsometer and fitting is applied to decrease a difference between the compared values in order to specify the thickness and the optical constant of the sample.