Abstract: The basic structure of an electrodialyzer consists of stacks in which cation exchange membranes, anion exchange membranes, gaskets (desalting cells and concentrating cells) are arranged alternately (figure 9.1 [Azechi, 1980]). Fastening frames are put on both outsides of the stack which is fastened up together through crossbars setting in the frames. The deformation of the membranes is prevented by regulating hydrostatic pressure in the fastening frames. Inlet manifold slots and outlet manifold slots are prepared at the bottoms and heads of the gaskets, respectively. Spacers are incorporated with the gaskets to prevent the contact of cation exchange membranes with anion exchange membranes and to mix the solution. Many stacks are arranged through the fastening frames, and electrode cells are put on both ends of the electrodialyzer, which are fastened by a press putting on the outsides of electrode cells (figure 9.2). An electrolyte solution to be desalted is supplied from solution feeding frames to entrance manifolds, flows through entrance slots, current passing portions and exit slots, and discharged from exit manifolds to the outside of the stack (figures 9.1 and 9.2). A concentrated solution is supplied to concentrating cells with a part of a feeding solution in a circulation flow system, and discharged to the outside of the stack through an over flow extracting system.