PURPOSE: To make it possible to separate and remove reactive dye from an aqueous solution containing the reactive dye early and efficiently by adding alkaline earth metal salts to an aqueous solution with pH higher than a specified value which contains the reactive dye and separating a deposited substance.

CONSTITUTION: Alkaline earth metal salts (e.g. calcium chloride) are added to an aqueous solution with pH higher than 9 which contains reactive dye to separate a deposited substance. The addition amount of the alkaline earth metal salts is preferably 10 to 60g/liter in terms of anhydride. This treatment method solves pending issues peculiar to a conventional technology such as deterioration of a removal rate of pollutant due to the insufficient quantity of an organic coagulation sedimentation agent, which is attributed to a variation of pollutant concentration in waste water, or an increase in BOD, COD due to the excessive removal rate, and some problems of the removal rate of pollutant due to the use of a strongly acidic inorganic coagulation sedimentation agent as well as the difficulty of controlling the pH value in such case. Further, it is possible to separate and remove reactive dye from an aqueous solution containing the reactive dye easily and efficiently, without using a substance which falls under the provision of the enforcement ordinance of the water quality contamination inhibitive law for human health and life environments as a treatment chemical.