Waste liquid treatment method and waste liquid treatment device

The invention relates to a waste liquid treatment method and a waste liquid treatment apparatus. The waste liquid treatment method comprises: a first separation step for taking out first treated water from a waste liquid containing a coating material; an oxygenolysis step for oxygenolyzing organic substances contained in the first treated water; and a second separation step in which the second treated water is taken out. The oxygenolysis step includes: a first treatment step in which the first treatment is performed until the temperature of the first treated water reaches the first temperature; a cooling step for cooling the first treated water; and a second treatment step in which a second treatment is performed under conditions in which the temperature of the first treated water is made to reach a second temperature lower than the first temperature in the cooling step, the first treatment and the second treatment being a treatment in which organic substances contained in the first treated water that has bee