Abstract: A method and flocculant for water treatment in which the highly polymerized silicic acid and a water-soluble salt of a metal capable of forming a hydroxide in water such as aluminum are added to the water to be treated. The highly polymerized silicic acid can be prepared by polymerizing water glass. In the case where the highly polymerized silicic acid and the water-soluble metal salt are prepared as a flocculant in the form of aqueous solution before adding to the water to be treated, the flocculant is maintained at a pH value of not less than about 2 or not lower than about 11. The flocculant may be a silicic acid solution of which the gel time is prolonged by being added with a substance capable to generate ferrice ion in the solution so that the flocculant can maintain high aggregation ability after being stored for a long time. The silicic acid solution may be polymerized before being added with such ferric ion generating substance.