We propose a sinusoidal wavelength-scanning interferometer for measuring thickness and surface profile of a thin film. The interference signal contains phase modulation amplitude Z and phase α which are related to the positions and profiles of the reflecting surfaces, respectively. By reducing the difference between the detected signal and the estimated signal using the multidimensional nonlinear least-squares algorithm, we estimate values of Z and α. Experimental results show that the front and rear surfaces of a silica glass plate of 20 μm-thickness could be measured with an error less than 10 nm. © 2005 The Optical Society of Japan.