An on-line turbidity unit is provided to discharge foreign substances by forming a drain valve in a tube connecting between a bubble removing container and a cell block. In an on-line turbidity unit, a case(10) is capable of receiving. A regulator(20) supplies sampling water with constant flow rate and speed. A bubble removing part(30) has a tube pipe(22) so as to introduce the sampling water passed through the regulator, an opening part at the center of the upper part, a bubble removing container(32) having a drainage hole penetrating a lower part, and a cover(33) formed at the center of an air hole. A drain part(40) includes a tube pipe(42) combined with a drain valve(41) for discharging foreign substances A cell block part(50) has a tube pipe(53) flowing the overflown sample water into a filtration plant through the lower part of the case. A cleaning part(60) includes a cleaning brush eccentrically rotating at the lower part of a guide. An amplifier(70) amplifies and converts a measured value of a sensor p
