A shape determining device includes first and second homodyne interferometers respectively provided for front and back surfaces of an object to be measured and a thickness distribution calculator that calculates a thickness distribution of the object based on intensities of first and second interference light beams respectively detected by the first and second homodyne interferometers for the front and back surfaces of the object at a plurality of measurement sites. The thickness distribution calculator calculates, for each interference light beam for which the intensity is detected by the first and second homodyne interferometers, a phase difference between the polarization components of a corresponding reference light beam and a corresponding object light beam in a corresponding non-interference light beam based on the intensity of the interference light beam, and calculates the thickness distribution based on a distribution of the calculated phase differences.