A thin‐film optical waveguide with self‐focusing caused by controlled distribution in the thickness was made of nitrocellulose and suspended in air. Undulation of the guided light beam was observed in the film, as well as in the self‐focusing optical fiber, when a He&sngbnd;Ne laser beam was injected at a place apart from the thickest center of the film. From this experiment, it has been shown that the transverse focusing effect in thin‐film optical‐wave transmission is obtained by the distribution in thickness of the film.