To coagulate and separate lignin from a black liquor containing no sulfide discharged from a pulp cooking process by a water treatment technology and to remove an organic matter remaining in a filtrate to collect water, an acid, and caustic soda.

In a method for treating the black liquor, a mineral acid and, if necessary, a dilution water is added to the black liquor to adjust pH to 1-7, and then a coagulant is added thereto to filter and separate coagulated lignin. The filtrate is brought into contact with ozone to oxidatively decompose the organic matter in the filtrate, and the remaining organic matter is adsorbed and removed by activated carbon.