A system including an index assigning section estimating the thickness of a thin film based on the intensity of light reflected on a substrate and a theoretical formula expressing a relationship between the thickness of the thin film and the intensity of an interference light when the spectrum is obtained for the first time, and assigning indexes for each candidate value for the layer thickness; a layer thickness wide-range estimating section estimating the layer thickness within the range of the value of a layer thickness wide-range estimation width including the previously estimated value of the layer thickness based on the theoretical formula; a selecting section selecting an index from the indexes; a determining section determining a layer thickness wide-range estimation result; and a layer thickness determining section determining the thickness of the thin film based on the theoretical formula after calculating the layer thickness wide-range estimation result.