A method for polishing a thin film formed on a substrate includes planarizing a thin film formed on a reference substrate by a CMP process such that the thin film remains on the reference substrate. After the planarizing, the thin film is cleaned, and then values of .DELTA. and .PSI. with respect to the cleaned thin film are measured by ellipsometry. A physical property of the thin film is determined based on the .DELTA. and .PSI. which have been measured by ellipsometry, and a polishing condition for an other substrate having a thin film to be polished is set based on physical property data which are obtained by the determining of the physical property.