Abstract: This invention provides a method of removing toxic substances such as fluorine, boron, copper, lead, arsenic, hexavalent chromium, and cadmium from strongly acid wastewater safely and without fail. The method comprises the steps of (i) adding hydroxyapatite 11 to strongly acid wastewater 1A to precipitate its toxic substances as solid constituents, (ii) adding an alkaline substance 12 to the wastewater 1A to neutralize it, (iii) adding a coagulant 13 to the wastewater 1A to coagulate the precipitated solid constituents, and (iv) removing the coagulated solid constituents from the wastewater 1A. According to this method, toxic substances in the strongly acid wastewater 1A can be fixed, or immobilized, effectively and removed safely. Besides, because the strongly acid wastewater 1A can be treated at normal temperature without heating it in any step, the wastewater 1A can be treated safely.