To provide a method not for detecting only an interface between a sludge accumulation layer and a supernatant water phase but capable of detecting also an interface between each layer in the sludge accumulation layer, and to provide a management method of a solid-liquid separation tank using the method.

Sludge sediments gradually in the solid-liquid separation tank 20 to form the sludge accumulation layer. In the sludge accumulation layer, a free sedimentation layer is formed on the uppermost part, and a coagulating sedimentation layer, a consolidation sedimentation layer and a concentration layer are formed successively on the lower side. An interface level meter for detecting the interface between each layer is used, and reception signal intensities are read at five times or more in 1-60 seconds, and the average value is used as the reception signal intensity. The interface between each layer is determined by using the data.