Abstract: Seawater was supplied to desalting cells in an unit-cell type electrodialyzer in Table 7.1 (Tanaka, 2005). The apparatus was incorporated with Selemion CMV/AST membranes and cross piece separators, and electrodialyzed at the linear velocity of 3 cm s-1 and current density of 2 A dm-2. The solution velocity distribution in stacks was evaluated by measuring the solution volume Q flowing out of the stacks. After that, the stacks were disassembled and washed, and the solution velocity distribution was measured again in the same manner. Frequencies of stacks Nstack are plotted against solution volume ratio ξ = (Q – Q-)/Q-, (Q-: the average solution volume in a stack), and shown in figure 7.1 which is equated with the normal distribution. The standard deviation of the normal distribution ζ is measured as 0.037. Next, the solution velocity distribution in desalting cells in one of the stacks before assembly was evaluated by injecting coloring liquid into the inlets of each desalting cell and measuring elapsed time until the coloring liquid appears in the outlets of the desalting cells. The average linear velocity u was adjusted to 3 cm s-1. The frequencies of desalting cells Ncell are plotted against the solution velocity ratio ξ = (u - u-)/u-, and shown in figure 7.2 which is also equated with the normal distribution and ζ is measured as 0.078.