Abstract: This wastewater treatment system is constituted by being provided with a solid-liquid separator 2 that separates the liquid portion and the solid portion from organic wastewater, a first anaerobic treatment tank 4 that carries out anaerobic treatment of the liquid portion separated by the solid-liquid separator 2 and obtains processed water that has passed through a filter mechanism 3 from sludge, a second anaerobic treatment tank 5 that carries out anaerobic treatment of the solid portion separated by the solid-liquid separator 2, and a sludge supply mechanism 6 that supplies the sludge in the second anaerobic treatment tank 5 to the first anaerobic treatment tank 4, and the filter mechanism functions stably without increasing the size of the anaerobic treatment tanks.