A case of the suspected food poisoning related to deer meat occurred in December, 2011 in Shiga prefecture in Japan. Four of 18 people showed transient diarrhea, abdominal pain, nausea, and vomiting within 5 to 16 hr after eating. No typical food poisonous bacteria and viruses were detected in the food samples. Parasitological tests were performed on the deer meat, and the Ministry of Health, Labour and Welfare of Japan for horsemeat food poisoning were officially notified. A 1,100-bp DNA fragment was amplified by PCR from three slices of the deer meat, suggesting the presence of Sarcocystis sp. Cysts and bradyzoites were detected in the specimens of the deer meat. Immunohistochemical staining of the cysts detected in the deer meat with an antibody against the toxic 15 kDa protein of S. fayeri showed a positive reaction. This indicated that a similar toxic protein originating from Sarcocystis cysts was present in the deer meat. This suggested that the deer meat containing Sarcocystis cysts was the causative food in these cases of food poisoning.