To exterminate red tide by solving such problems that conventional red tide exterminating agents which are given by using tuff containing zeolite, montmorillonite, etc., as main components and a powder of a mineral, such as acid clay, take enormous costs and efforts, because the conventional agents are required to be scattered in a considerably large amount, and conventional procedures, for which surfactants having relatively low toxicity to fishes are used, take costs, because the specified surfactants are required to be used, and by using the volcanic ash which is obtained at a low cost in a large amount, and to effectively utilize the volcanic ash which is a cause of environmental pollution.

A red tide exterminating agent is given by adding 4-6 pts.mass of the volcanic acid to 1 pt.mass of a precipitating agent comprising a gypsum component, an aluminum inorganic salt, an alkali metal carbonate, cement, and a polymeric flocculating agent. A red tide exterminating method uses the red tide exterminating agent.