To collect/obtain useful materials in seawater or the like and to desalinate seawater by concentrating the ions in seawater, which are permeated through a diaphragm 2 and moved toward the electrodes, in the vicinity of each electrode 1 by impressing the prescribed voltage between the electrodes 1 covered with an insulator by making good use of electrically attracting force.

Since the surfaces of the electrodes are covered with the insulator, the ions moved in the solution can be collected around each electrode and withdrawn as the concentrated ion-containing aqueous solution. Since electric power is not consumed so much, seawater or the like can be concentrated or desalinated at a low cost.