A polishing method capable of obtaining an accurate thickness of a silicon layer during polishing of a substrate and determining an accurate polishing end point of the substrate based on the thickness of the silicon layer obtained. The method includes: calculating relative reflectance by dividing the measured intensity of the infrared ray by predetermined reference intensity; producing spectral waveform representing relationship between the relative reflectance and wavelength of the infrared ray; performing a Fourier transform process on the spectral waveform to determine a thickness of the silicon layer and a corresponding strength of frequency component; and determining a polishing end point of the substrate based on a point of time when the determined thickness of the silicon layer has reached a predetermined target value.