Sciatic hernia and early-stage appendiceal adenocarcinoma are rare disorders. We report herein a case of an early stage of appendiceal adenocarcinoma found incidentally during an operation for sciatic hernia. An 80-year-old woman was admitted to our hospital with abdominal distension. Abdominal computed tomography showed a small bowel obstruction and a small bowel loop through the right sciatic foramen outside the pelvic cavity. During an operation for the repair of the sciatic hernia, a deformed appendix was found incidentally and a simple appendectomy was also performed. Histological examination of the excised appendix showed well-differentiated adenocarcinoma confined to the mucosal layer in a tubulo-villous adenoma. At 13-month follow-up, the patient is doing well without evidence of recurrence or metastasis.