Phosphate ion, which is a cause of metabolic bone disease, is excessively accumulated in the blood of chronic dialysis patients, because many of dialysis membranes have negative charge, which inhibit removal of phosphate ion from their blood. The ability to remove phosphate ion can be evaluated by membrane charge. The electrical charge of the membrane affects the rate of ion permeability, and the phosphate ion is no exception. The membrane charge of sheet dialysis membranes can be readily evaluated by measuring the membrane potential and calculating the effective charge density. Accordingly a study was made of the influence on the phosphate ion permeability exerted by the membrane charge of sheet membranes composed of the same material as the hollow-fiber membranes used in hemodialysis. The diffusive permeability of phosphate ion was greater due to the result that the higher the ionic strength in the solution, the higher the absolute ion mobility. On the other hand, because of their shape, it is impossible to measure the membrane potential of hollow-fiber membranes using conventional techniques, making it difficult to determine the effective charge density. The authors therefore, have proposed a novel technique to enable the measurement of membrane potential in hollow-fiber membranes, and have studied the influence exerted on the phosphate ion permeability by the effective charge density calculated by this technique. The phosphate ion permeability increases when the following situations occur: (1) the smaller the absolute value of effective charge density, (2) the greater the ionic strength of the solution, and (3) the greater the concentration of plasma proteins. This agrees with the theoretical results. By calculating the effective charge density from the membrane potential of the hollow-fiber membranes, as measured by the technique proposed here, it is possible to evaluate the rate of phosphate ion permeability in such membranes.