A film thickness measurement apparatus includes a measurement light source that supplies measurement light containing a measurement light component with a first wavelength and a measurement light component with a second wavelength to a measuring object, a spectroscopic optical system that decomposes interfering light of reflected light from the upper surface and reflected light from the lower surface of the measuring object into an interfering light component with the first wavelength and an interfering light component with the second wavelength, photodetectors that detect intensities of the first and second interfering light components at each time point, and a film thickness analysis section that obtains a temporal change in film thickness of the measuring object based on a phase difference between a first phase in a temporal change in detected intensity of the first interfering light component and a second phase in a temporal change in detected intensity of the second interfering light component.