The invention provides metal-silica polymeric flocculants having high coagulation performance which are used for water treatment, particularly to remove suspended materials and other impurities from irrigation water, waste water or the like. A silica sol having a SiO.sub.2 concentration of from 100 to 200 g/L and a viscosity of less than 6 cp is prepared by colliding an inorganic acid aqueous solution with a silicate aqueous solution at each rate of 5 m/sec or more. The silica sol is aged to make an aged silica sol having a viscosity of 6 to 30 cp, and the aged silica sol is mixed with a water-soluble metal salt such as ferric chloride, to prepare the flocculants.