Provided are a method and an apparatus that enable rapid and automatic determination of the coagulant injection rate in a process of water treatment through coagulation and sedimentation. Using a coagulation analyzer comprising sample tanks 1A to 1D each for keeping a predetermined amount of raw water, a water supply pump 7, water supply/discharge valves 4, 6 for raw water and washing water, mixers 3A to 3D, a coagulant injection unit 21, and a detector 30 for determining the particle size and the particle number of flocs, the time within which the coagulant 20 injected into the sample tanks is dispersed by mixing and the particles begin to agglomerate (agglomeration start time) is determined, and based on the thus-determined agglomeration start time, the coagulant injection rate is determined, or the amount of the coagulant to be injected is controlled.