To provide a waste water treatment agent which shows stable performance even at low temperatures and by which a small amount of a water-soluble contaminating material in waste water can be removed.

This waste water treatment agent is obtained by mixing polychlorinated aluminum being a flocculating agent different in molecular weight with aluminum chloride. When an aqueous solution of this waste water treating agent is added to the waste water containing a surfactant, the surfactant in the waste water is flocculated and precipitated as large flocks. When the mixing ratio of polychlorinated aluminum with aluminum chloride is 4:1 to 6:1 by weight in terms of aluminum oxide, satisfactory flocculating performance can be obtained even when a small amount of this waste water treating agent is added to the waste water of low temperature and preservation stability of the aqueous solution of this waste water treatment agent can be enhanced.