This water softening device (201) is provided with: a negative electrode (8) and a positive electrode (9); at least one water-splitting ion exchange membrane (1) that has a positive ion exchange surface (1a) on one surface, has a negative ion exchange surface (1b) on the other surface, and is layered between the negative electrode and the positive electrode in a manner so that the negative ion exchange surface faces the positive electrode and the positive ion exchange surface faces the negative electrode; and a casing (7) that houses the negative electrode, the positive electrode, and the at least one water-splitting ion exchange membrane. The casing has: a first entrance (10) and a first exit (11) that are respectively the entrance and exit for water for processing; and a second entrance (12) and second exit (13) that are respectively the entrance and exit for water for regeneration. The differential pressure of the water for processing between the first entrance and the first exit is greater than the differential pressure of the water for regeneration between the second entrance and the second exit at the same flow rate as that of the water for processing.