Provided are: a distributor capable of draining liquid uniformly from an entire opening without the opening being blocked by sludge, and with minimal pressure loss; a settling tank which uses the distributor; and a method for operating the settling tank. This rectangular frame-shaped distributor (10), which has straight tubular sides (11-14) is disposed at the lower part of a settling tank (1). A liquid drainage opening (16) extending in the longitudinal direction of the distributor (10) is provided in the bottom surface of the distributor (10). The open angle (theta) of the opening (16) with respect to the axis of the tube is 60-180 DEG, and preferably 90-150 DEG.