Membrane pair characteristics of commercially available ion-exchange membranes are measured by changing current density and seawater temperature supplied to the electrodialyzer. The hydraulic permeabilities (leading parameter) for three types of commercially available membranes are almost the same, and their averages are expressed by the empirical function of temperature. Hydraulic osmosis is predominant at lower current density and electro-osmosis is predominant at larger current density. The influence of temperature and salt concentration on the physical properties of saline water, such as solution density, specific conductance, and NaCl activity coefficient, is expressed by empirical equations. Ionic constituents in a concentrated solution are expressed by empirical equations. Electric current screening ratio of a spacer is defined and calculated. Direct current electric resistance of a membrane pair is calculated, and it is predominant over that of a desalting cell and a concentrating cell. It is necessary to decrease electric resistance of an ion-exchange membrane for reducing energy consumption in a salt manufacturing process.