The specification discloses a novel calcium silicate water-treating agent exhibiting a superior ability to remove various kinds of phosphorus compounds or other environmental pollutants contained in wastewater, the calcium silicate water treating agent comprising a burned product which is prepared by burning a mixture containing as main constituents calcium compound, thermally decomposable to calcium oxide (CaO), and SiO.sub.2 -containing compound at a temperature of 1300.degree. to 1600.degree. C., the molar ratio of CaO/SiO.sub.2 of the resultant burned product being in the range of 1.5 to 5.0, preferably 2.0 to 4.0. In this water treating agent, SiO.sub.2 component may be partially replaced by Al.sub.2 O.sub.3 and/or Fe.sub.2 O.sub.3. A further water treating agent is prepared by heating the hydrate of the burned product prepared above at a temperature of 50.degree. to 700.degree. C. By using the thus obtained water treating agents, water treatment can be carried out with a high efficiency, thereby the treatment time, labor and cost can be considerably saved.