Three cases of Diphyllobothriasis, which is caused by one of the commonest parasites transmitted by the fish in Japan, have recently been encounters. In all three cases, chief complaints were abdominal pain and diarrhea. The patients discovered a chain of tapeworm proglottids discharged spontaneously on their anus and brought them for identification. The first case was in a 25-years-old man. After admission, he used a laxative, and two strobila, one 140cm long with a scolex, and the other 180cm long without a scolex. The second case was in a 59-years-old woman. After admission, she was treated with Gastrografin (100ml) by gastrointestinal tract, and expelled a strobila without a scolex, 880cm. The third patient was treated only with praziquantel. Ultimately, praziquantel was administered orally to all three patients. We confirmed the absence of tapeworm eggs in the patients' stools two months later.