PURPOSE: To produce high-purity magnesium hydroxide from crude magnesium hydroxide obtained economically from sea water and quick lime or slaked lime.

CONSTITUTION: Crude magnesium hydroxide containing impurities is made to react with an aqueous solution of a water-soluble calcium salt and carbon dioxide gas to form an aqueous solution of a magnesium salt and precipitate of calcium carbonate. The impurities are transferred to the precipitate by this process. The precipitate is separated by filtration, and the obtained aqueous solution of magnesium salt is made to react with ammonia to obtain a magnesium hydroxide slurry and an aqueous solution of an ammonium salt. The slurry of magnesium hydroxide is separated by filtration to obtain high-purity magnesium hydroxide.