PURPOSE: To provide a method for redesalting a concentrated drained water discharged from a desalting process with a concurrent action to prevent a sparingly soluble salt from depositing, and further, concentrating the drained water beyond an ordinary concentration level.

CONSTITUTION: When dialyzing a concentrated drained water discharged from a desalting process in an electric dialysis tank in which a univalent ion selective transmission membrane is built, a desalting water supplied to the concentration chamber of the electric dialysis tank, e.g. water refined by the desalting process, is used.