PURPOSE: To obtain an excellent filling agent for chromatography by making phosphoric acid or a calcium salt thereof react with a calcium compound in an organic solvent to form a specified hydroxy apatite easily with a better reproducibility.

CONSTITUTION: Phosphoric acid or a calcium salt thereof such as calcium hydrogenphosphate 2 water (CaHPO4.2H2O) is made to react with a calcium compound such as calcium hydroxide [Ca(OH)2] in an organic solvent, for example, n-hexane and water at about 60°C. Here, Ca/P in the number of atoms is set at about 1.3W1.9. After about 2hr of reaction, an reactant is filtered and dewatered to obtain hydroxy apatite [Ca10(PO4)6(OH)2]. The apatite thus obtained is less than 50m2/g and forms a hexagonal prism-shaped assembly. Thus, the reaction in the organic solvent enables the manufacture of a filling agent for chromatography of hydroxy apatite excellent in the separation performance in a constant character with a better reproducibility.