Disclosed is a method for measuring a thickness of a subjecting layer attached on a base layer by means of an interferometer, which includes the steps of: obtaining a correlation equation of a phase difference with respect to thicknesses of sample layers, the thicknesses being different from each other, the sample layers being made from a material substantially equal to a material of the subjecting layer; obtaining a first interference signal with respect to an optical axial direction incident to the base layer at a boundary surface between an air layer and the base layer; obtaining a second interference signal with respect to the optical axial direction at a boundary surface between the subjecting layer and the base layer; obtaining a phase difference between a phase of the first interference signal and a phase of the second interference signal at respective heights substantially equal to each other with respect to the optical axial direction; and determining a thickness of the subjecting layer by inserting the phase difference into the correlation equation.