Abstract: In a water purification plant, where a coagulant is dosed into raw water to coagulate turbidity-causing particles suspended in the raw water to form floc, and the water containing the floc is led to a sedimentation basin to separate the floc by sedimentation, thereby purifying the raw water, a coagulation effect is quickly detected by by-passing a portion of the water containing the floc and flowing to the sedimentation basin through a filter column of an apparatus for detecting a coagulation effect, the filter column being filled with a filter layer of filter medium, detecting a filtration pressure at the column side at a specific level of the filter layer at a specific time interval, monitoring a change in a rate of change in the filtration pressures to detect an improper dosage of the coagulant in correlation to the change in the rate of change in the filtration pressure, and outputting an alarm signal to a coagulant dosage control system, when the detected coagulation effect is detected improper.