The electrical potential differences were measured at various points in an organic phase of a water-oil-water liquid membrane system in which self-oscillations of an electrical potential difference occurred across a three-phases-liquid membrane by using an H-shaped cell. The electrical potential difference oscillated in the direction through the two aqueous phases, but not in its vertical direction. The amplitude of the self-oscillation greatly changed around the place where the horizontal tube was connected to the vertical tube, which had no surfactant.