Abstract: The thickness of a thin layer is measured in which a patterned lower layer is formed on a substrate, and an upper layer covering the lower layer is formed. The lower layer is modeled, and then the thickness of the upper layer is measured. When modeling the lower layer, the lower layer is set as a predetermined material layer, and then the material layer is fit into the lower layer using a harmonic oscillator model using an asorptivity, a refractive index, and a thickness of the material layer as parameters. Accordingly, the thickness of a thin layer formed in a cell area can be directly measured in real-time. Also, the thickness uniformity of the thin layer can be measured in each shot area or each chip area. Furthermore, the thickness of a portion of the thin layer formed in a chip at the edge of wafer can be measured. As a result, the method can be applied to improve processes and analyze problems of the processes.