PROBLEM TO BE SOLVED: To provide a method for improving the underwater environment by which the method can be carried out at a low cost in a sea area where a withered seashore or red tide is actually caused or there is a fear of causing the withered seashore or red tide or a sea area where decline or disappearance of the seaweed growing environment due to a cause other than the withered seashore is actually caused or there is a fear of causing the decline or disappearance and excellent effects on prevention of occurrence of the withered seashore or prevention of the red tide, creation of seaweed beds or repair, etc., of seaweed growing environment are obtained. SOLUTION: This method for improving the underwater environment comprises sinking a granulated blast furnace slag in the sea area where the withered seashore or red tide is actually caused or there is a fear of causing the withered seashore or red tide or the sea area where the decline or disappearance of seaweed growing environment due to a cause other than the withered seashore is actually caused or there is a fear of causing the decline or disappearance. Thereby, extremely effective countermeasures for preventing the withered seashore or countermeasures for red tide or the creation of seaweed beds or the repair of the seaweed bed growing environment can be realized at a low cost. The method is characterized by installing the granulated blast furnace slag as a silicate ion releasing source in water.
