The present invention provides a thin film property measuring method using a spectroscopic ellipsometer. With the measuring method, a model including a combination of the film thickness, complex refractive index, or the like, of each layer is formed, and fitting is made for the measured spectra and the spectra calculated based upon the model, with the model and the incident angle being modified over a predetermined number of repetitions, thereby determining the structure, the wavelength dependency of the dielectric constant, and the composition ratio, of a thin film including a compound semiconductor layer on a substrate. Furthermore, new approximate calculation is employed in the present invention, thereby enabling the concentration of the atom of interest contained in polycrystalline compound semiconductor to be calculated.