Lignin and alkali are efficiently sorted out of alkaline black liquor discharged from the paper manufacturing/pulp and paper manufacturing industries and the treated water is made as purified water.Acid is added to the black liquor and its pH is adjusted to 2.5 to 3.5. An aggregating agent is added to have lignin contained in the black liquor settled and the black liquor is separated into lignin and clean water. Ozone gas, preferably in the micro bubble state, is given to the clean water for contact reaction.