Abstract: Two sub-desalination chambers d1 and d2 are defined by a cation exchange membrane 3 on one side, an anion exchange membrane 4 on the other side, and an intermediate ion exchange membrane 5 in between and are filled with ion exchange materials 8 to construct a desalination chamber D. Concentrating chambers 1 are provided via the cation exchange membrane 3 and the anion exchange membrane 4 on both sides of the desalination chamber D. The desalination chambers D and the concentrating chambers 1 are provided between an anode 7 and a cathode 6. While a voltage is applied between the anode 7 and the cathode 6, water is supplied to one of the two sub-desalination chambers d2 and then, water discharged from the first sub-desalination chamber d2 is supplied to a second sub-desalination chamber d1. Concentrate water is supplied to the concentrate chambers.