To provide a simple operation method for an anaerobic treatment tank capable of continuing anaerobic treatment over a long period of time without hindrance even if scum is floated on the surface of water to be treated during anaerobic treatment.

The operation method for the anaerobic treatment tank 10 used in a water treatment system using both of anaerobic treatment and aerobic treatment has a scum treatment process wherein a stirrer 3 having a screw for stirring the water to be treated in the anaerobic treatment tank is provided in the anaerobic treatment tank and, before the surface F1 of the water to be treated in the anaerobic treatment tank is wholly covered with scum S or before the flow of scum floating on the surface of the water to be treated is stopped, the screw of the stirrer is reversely rotated to perform treatment for reducing the amount of scum floating on the surface of the water to be treated.