To provide a water purifier, containing: a diluting unit configured to bring targeted water for purification into contact with an aqueous solution containing a volatile solute and a polymer via a semi-permeable membrane so as to separate water from the targeted water by the semi-permeable membrane, and configured to dilute the aqueous solution with the separated water; a separating unit configured to separate the volatile solute and the polymer from the diluted aqueous solution obtained by the diluting unit so as to obtain purified water; and a dissolving unit configured to return the separated volatile solute by the separating unit to the aqueous solution containing the polymer, and to allow the separated volatile solute to dissolve in the aqueous solution containing the polymer.