We study phenomena called Ostwald ripening in a thin liquid film by a numerical method. Droplets such as oil-water system in a thin film disappear or grow slowly. We simulate these phenomena by the invariant finite difference scheme of the two-phase shallow water equations. We show that difference of chemical potential between surface of droplets and outside of droplets plays an important role, and we also evaluate the speed of the flow outside droplets by the reductive perturbation method.