Based on the principle of leaky waveguide, we made measurement of the refractive index of a cryolite film deposited on a K9 glass substrate. The accuracy is about 1 x 10-4, which is similar to that for a real waveguide. Possibility of decreasing the minimal measurable thickness of the film by insertion of a high index thin layer between the substrate and the film to be measured is discussed. When a white light source is used, we make judgement of waveguide excitation by observing the dark m-lines in the reflective light; in this way,the refractive of the cryolite film in the wavelength range from 450.0 to 650.0 nm is determined.