We experienced a case of perforated appendix by a fish bone that did not show obvious signs of inflammation. Patient was a 55-year-old male, who presented with a chief complaint of right lower quadrant abdominal pain. Physical examination suggested presence of appendicitis. The final diagnosis was made at the time of surgery as perforated appendix caused by a fish bone. It is quite rare to see a perforation of appendix by a fish bone swallowed by mistake. Eighteen such cases in including ours have been reported in Japan since 1989. Most of the cases were operated on with preoperative diagnosis of acute appendicitis of ileo-ceacal abscess with inflammatory signs, and only 3 cases had been correctly diagnosed preoperatively. In 2 cases blood study and image studies were normal. In this regard we believe careful physical examination of the patients are very important. A review of the literatures were done.