PROBLEM TO BE SOLVED: To provide a bactericidal agent derived from a natural product having a high bactericidal level. SOLUTION: A primary firing step of calcining oyster shells under predetermined conditions to obtain a primary calcined product； a step of cooling the primary calcined product； and a secondary firing of the cooled primary calcined product under predetermined conditions. A calcined oyster shell powder having a calcium oxide content of 80% by mass or more based on the total mass of the secondary calcined product, which is obtained through a secondary calcining step of obtaining the product. [Selection diagram] None【課題】高い殺菌レベルを有する天然物由来の殺菌剤を提供する。【解決手段】カキ殻を所定条件で焼成して一次焼成物を得る一次焼成工程と；当該一次焼成物を冷却させる工程と；冷却された当該一次焼成物を所定条件で焼成して二次焼成物を得る二次焼成工程と；を経て得られたことを特徴とする、二次焼成物の全質量を基準として酸化カルシウムの含有量が８０質量％以上である焼成カキ殻粉末。【選択図】なし
