We report a fatal case of hypermagnesemia caused by oral ingestion of milk containing magnesium chloride as a folk remedy. The autopsy of the 75-year-old woman showed no injury or illness as the cause of death. Neither alcohol nor controlled drugs were found by the toxicological analysis. Her posthumous serum magnesium concentration was 10.2 mg/dL. The concentration of magnesium in serum was higher than that of the previous reports. Therefore, the cause of death was considered to be hypermagnesemia. Recently, the prescription drugs, over-the-counter drugs, and health foods containing magnesium are consumed as supplements in the world, and their potential toxicity should be recognized.