In a settling tank of the invention, treated water has good quality and whose operation is stable over a long period. A settling tank includes a tank body having a circular-cylindrical shape whose axial direction corresponds to the vertical direction, a distributor provided in a lower part (near the bottom) of the tank body, an outlet provided in a middle part or a part slightly above the middle part of a side face of the tank body, a receiving tank communicating with the tank body via the outlet, a stirring device provided in the tank body, a trough for extracting treated water provided in an upper part of the tank body, a sludge discharge port through which concentrated sludge is extracted from a lower part of the receiving tank, and so forth. A stirring member is provided below the distributor.