Abstract: Original seawater is supplied to a pre-treatment equipment, and a flocculated material produced by exerting a magnetic force and injecting ozone is removed by filtration. The seawater pretreated by the pre-treatment equipment is supplied to a reverse osmosis membrane module. Concentrated salt water discharged from the reverse osmosis membrane module is supplied to an electrodialyser unit. The concentrated salt water discharged from the electrodialyser unit is supplied to an evaporator. In the evaporator, the concentrated salt water is evaporated to dry solid salt. A part or all of desalted water discharged from the electrodialyser unit is returned to the reverse osmosis membrane module. Permeated water discharged from the reverse osmosis membrane module and evaporated water obtained by the evaporator are supplied as potable water.