Method of measuring thickness of thin film

Disclosed is a method of measuring thickness of a thin film, the method comprising the operations of: acquiring reflectance, supposing the refraction, performing conversion, determining an error, extracting a phase, restoring the phase and calculating the thickness. In the operation of acquiring reflectance, a light-intensity signal of a thin film is acquired and the reflectance of the thin film is acquired based on the light-intensity signal. In the operation of supposing the refraction, refraction of air and refraction of the thin film are supposed to be a real number, and refraction of a substrate formed with the thin film is supposed to be a complex number. In the operation of performing conversion, the reflectance is converted into regular reflectance. In the operation of determining an error, it is determined whether a first difference value indicating difference between a maximum value of the converted regular reflectance and a first reference value or a second reference value indicating difference bet