A spatial pattern of the electric potential formed by a characean internodal cell is investigated experimentally. A multi-electrode measuring system is used to pursue the dynamic properties of the pattern. It is shown that periodic low-potential regions are formed after the entire transient increase over the cell when the light is imposed on. Under constant illumination an electric oscillation is found. These experimental data are also discussed by the use of a spectrum analysis; the formation of the pattern is a bifurcation phenomenon of soft-mode instability realizing far from equilibrium, accompanied by sharpening of spectrum of a particular wave mode. A long-range coherence extending over 1 cm is also shown to exist in the high-potential region, where protons are extruded by H<SUP>+</SUP>-ATPase molecules existing within the membrane.