A water treatment device in which a treatment target water introduction portion is provided in an upper portion of a receptacle, and the water treatment device has a space in which conductive porous members are not disposed between the conductive porous members and the treatment target water introduction portion in the receptacle. When a backwashing fluid is introduced from a lower portion of the receptacle in a desorption, the conductive porous members flow and are agitated owing to the backwashing fluid, because of which a desorption of ions adsorbed to the conductive porous members is promoted. Grains of the agitated conductive porous members collide with other grains of the conductive porous members or with electrodes or a separator, whereby scale and a biofilm appearing on surfaces of the grains of the conductive porous members, the electrodes, or the separator can be removed, and desalination efficiency can be maintained.