PROBLEM TO BE SOLVED: To provide a method for exterminating ectoparasites which is parasitic on cultured fishes.SOLUTION: The method for exterminating ectoparasites which is parasitic on cultured fishes to be provided is characterized by: setting an electrolytic ozone generator near a farm; electrolyzing raw water by the electrolytic ozone generator to generate electrolysis ozone water; storing the generated electrolysis ozone water within an electrolytic ozone water tank; and loading cultured fishes into the electrolytic ozone water tank to exterminate ectoparasites which is parasitic on the cultured fishes.