A method of treating silicon powder-containing drainage water, comprising the steps of: including an inorganic flocculant composed of a silica sol-water-soluble metal salt having a molar ratio of silicon to metal of 0.05 to 3.0 into drainage water containing silicon powders to ensure that the metal concentration becomes 15 to 300 mg/L; and adding an organic polymer flocculant. In this method, the coagulation treatment of drainage water containing fine silicon powders can be carried out.