1. A gap measuring method comprising the steps of: shifting a wavelength of light stepwise within a predetermined wavelength range to use the light for measuring a gap; taking an image of an interference fringe, which is generated owing to a gap provided in a measured object and the used light with the shifted wavelength, by a camera correspondingly to each step of the shifted wavelength; obtaining a change in the intensity of the images of the interference fringes taken by said camera at each of predetermined positions of each image, over said predetermined wavelength range; and determining gap values of a plurality of points of the gap based on the obtained changes in the density.