An object of the present invention is to provide a red tide control agent having an immediate effect and greatly improved red tide control efficiency, and a red tide control method using the same. In order to achieve the object, the present invention provides a red tide control agent containing magnesium oxide having an average particle size of 0.1-1 [mu]m and a BET specific surface area of 15 m2/g or more. Also, provided is a red tide control method characterized in that a red tide control agent containing magnesium oxide having an average particle size of 0.1-1 [mu]m and a BET specific surface area of 15 m2/g or more is sprayed over a sea area where red tide occurs.本發明提供一種具有速效性且大幅提高了紅潮之驅除效率之紅潮驅除劑及使用有其之紅潮驅除方法。 本發明之紅潮驅除劑，其含有平均粒徑為0.1〜10 μm且BET比表面積為15 m2/g以上之氧化鎂。又，本發明之紅潮驅除方法其在產生了紅潮之海域中散布含有平均粒徑為0.1〜10 μm且BET比表面積為15 m2/g以上之氧化鎂之紅潮驅除劑。
