PURPOSE: To obtain treating substance containing a component suitable for the quality improvement of water by a method wherein a natural shell fossil containing components of plankton or algae are ground to be dried naturally and further dried at specific temp. by hot air to be purified into a particulate form and this particulate material is further classified by air to be purified into a specific particle size.

CONSTITUTION: A natural shell fossil containing components of various nektons, planktons, algae or sea weeds composed of calcareous substance or silicate are ground into small particulate or rough sandy coarse mineral which is, in turn, dried at about 120-450°C by hot air through a rotary kiln A and a cooler B to be purified into a particulate form. This particulate is further ground into a powdery form through a ball mill C and a cyclone D to be finely purified into a particle size of about 0.10-50.00μm. By this method, treatment substance optimum to the quality improvement of water is obtained.