A computer simulation program was developed for a continuous saline water electrodialysis process. A practical-scale electrodialyzer was operated for concentrating seawater, and its performance was observed. The operating results of the electrodialyzer were computed by using the operating conditions as input for the program. The computed data were generally the same as the observed data. Thus, the program enables the semiquantitative discussion of the performance of a practical-scale electrodialyzer. In addition, (1) the current density distribution; (2) the electric resistances of a membrane pair, a desalting cell, and a concentrating cell; (3) the pressure drops in a desalting cell and a concentrating cell; and (4) the limiting current density of an electrodialyzer are discussed.