Methods for measuring a thickness of an object including acquiring at least one of a wavelength domain spectrum for an amplitude ratio (Ψ) and a phase difference (Δ) of reflected light from a film material, converting the wavelength domain spectrum into a 1/wavelength domain spectrum, acquiring a resulting spectrum by performing fast fourier transform (FFT) on the 1/wavelength domain spectrum, and measuring a thickness of the film material from the resulting spectrum may be provided.