Composite membranes composed of cation exchange membranes and polypyrrole, in which polypyrrole existed on one surface of the membrane as a thin layer, were prepared with ferric ion form cation exchange membranes and an aqueous pyrrole solution. After confirming the existence of the polypyrrole layer on the membrane surface (EPMA, conductivity, and micrograph), the following electrodialytic transport Properties of the membrane were evaluated : the electrical resistance of the membrane, the relative transport number between the alkali earth metal cations and sodium ions, the current efficiency, and voltage drop at the membrane. The permeation of calcium ions was markedly decreased by the formation of a polypyrrole layer on the cation exchange membranes in both cases where the layer faced the anode and the cathode compartments. It was confirmed by a decrease in the current efficiency during electrodialysis in the layer facing the cathode compartment that the polypyrrole layer has a cationic charge. The permeability coefficient of the neutral molecules through the membrane was markedly decreased by the formation of a layer on the cation exchange membrane. From these results, a decrease in the permeation of the alkali earth metal cations versus sodium ions was observed based on the synergistic effect of the sieving of divalent cations by a tight and rigid polypyrrole layer and the difference in the electrostatic repulsion force between the divalent cations and the sodium ions.