The multi-wavelength back-propagation (MWB) method enables to determine precisely the optical path different (OPD) longer than the optical wavelength from detecting the amplitude and phase of the interference signal for the multiple wavelengths. In this study, we demonstrate a 1-dimensional thickness profile measurement by the MWB and sinusoidal phase modulation (SPM) technique with a spectral interferometer. The OPD for the front and rear reflecting surfaces of a glass film with the thickness of 100 mu m are measured. The thickness profile is successfully measured with repeatability of 2 nm estimated from a standard error between 9 repeatedly-measured profiles.