PURPOSE: Oyster shells are heated at elevated temperature in the air for hours, then exposed to the air to enable the production of a calcium powder that can be used as an active calcium preparation.

CONSTITUTION: Oyster shells are crushed into appropriate sizes and heated in an electric oven or petroleum burner without isolation from the air at 1,500W 2,000°C for 10W12hr. Then, the product is exposed to the air for 5W10hr to be cooled and crushed into fine particles. The process according to the present invention enables ready and large-scale production of a starting material for a calcium preparation with high activity and high absorbability from intestinal tracts.