The motion of the front between thick and thin foam film formed in a vertical frame is studied both experimentally and theoretically. In the experiments we used sodium dodecyl sulfate solutions with added NaCl. The rate of motion of the front turns out to be constant for a given electrolyte concentration, Other important quantities that are measured are the thickness of the black film and the jump of the film tension, when the front reaches the bottom meniscus. The rate of the front turns out to be proportional to the jump of the film tension irrespective of the amount of added electrolyte. The theoretical model we developed allows quantitative interpretation of the experimental data. It is demonstrated that the motion of the front "thick-thin" film is accompanied with the formation of microscopic steady capillary waves in the transition zone. We show that the energy dissipation in the him is concentrated mainly into the latter part of the transition zone. The study can be helpful for the understanding of the mechanism of the foam films drainage as related to the stability of foams.