Our surveillance indicated the food-borne disease associated with Kudoa septempunctata has occurred on summer season. To elucidate the reasons of that, we investigated the temperature effect of food-borne disease associated with K. septempunctata. We continually purchased olive flounders in the same lot from the fish farm that was infected with K. septempunctata partly and determined the number of spores in olive flounder muscle. Both the positive ratio of K. septempunctata in olive flounder and the number of spores did not show the seasonal change from January to August. We discovered that the temperature of seawater in summer season was over 20℃. However, the positive ratio of K. septempunctata, the number of spores and the toxicity of K. septempunctata were not affected by high temperature of seawater. These results demonstrated that the temperature rise of sea water was not a reason why the frequency of the food-borne disease increases in summer season.