Provided is a method for treatment of water by flocculation precipitation, such that the concentration of particles does not drop in a flocculation step in a sludge-blanket vessel. The method for the flocculation treatment of source water (1) comprises an inorganic flocculant-injection step, a fine-flocculation step in a rapid mixing vessel, a flocculation step in a sludge-blanket vessel (5), anda separation-discharge step for precipitating, separating and discharging the flocculated particles, wherein the precipitated flocculated particles are returned to a step before the fine-flocculationstep to keep the particle concentration before the fine-flocculation step not lower than 7.5 mg/L so as to be suitable to an operation site, based on a preliminary experiment regarding change in the concentration of the particles in the sludge-blanket vessel (5) by injection to the source water (1).