A 57-year-old man was seen at the emergency department in our hospital because of right lower abdominal pain. Tenderness and abdominal guarding were recognized in the right lower quadrant. Abdominal CT scan showed a mass measuring 4cm in diameter with a high density spot in the center of the mass. Acute appendicitis caused by a coprolith was suspected and appendectomy was performed. A fish bone 1cm in length stuck in the appendiceal wall was seen in the resected specimen. The appendix sitnated distal to the fish bone showed a marked swelling. Microscopic examination revealed a cut on the appendiceal wall from where the fish bone might have stuck, severe inflammatory changes around the cut, acute appendicitis, and multiple diverticula lacking the muscularis propria. Thus, inflammatory obstruction of the appendix caused by the ingested fish bone was thought to contribute the pathogenesis of both acute appendicitis and diverticula of the appendix in the present case.