The purpose of the present invention is to provide a method for controlling saprolegniasis in culture water using chlorine dioxide that has lower toxicity and higher safety than bronopol. According to the present invention, saprolegniasis is controlled by adding a chlorite to culture water, said culture water having a pH value of 5.5-8.5 inclusive, to give a concentration in a range of 2.5-200 ppm inclusive in terms of effective chloride dioxide and then allowing to react for 60 minutes or longer. In this method, neither any organic acid nor any inorganic acid is added to the culture water. The culture water includes water for hatching and water for feeding. Also, the culture water includes both seawater and freshwater.
