Abstract: Provided is a method of treating wastewater, which can treat wastewater, particularly organic wastewater while saving labor and decreasing the environmental impact by improving deterioration in the purification performance due to clogging of the filter bed, which is regarded as a problem when purifying wastewater, particularly organic wastewater having high SS concentration, BOD, and COD by a constructed wetland and by decreasing increases in the area and the number of stages of wetland required due to an increase in the BOD and COD load. A method of treating wastewater provided comprises: an aggregate floc forming step of forming an aggregate floc by adding a polymer flocculant to wastewater; a solid-liquid separation step of obtaining separated water by solid-liquid separation of the aggregate floc; and a purification step of purifying the separated water by using a constructed wetland.