A 50-year-old man was admitted to the hospital because of right lower abdominal pain lasting for 10 days. Abdominal ultrasonography (US) revealed an extraluminal low echoic mass and abdominal CT scan showed a low density mass in the ileocecal region. Colonoscopy revealed deformity of the orifice of the appendix and reddish mucosa around the diverticula in the cecum. Although conservative treatment led to a shrinkage of the abscess cavity and reduced levels of inflammatory markers, the abdominal pain persisted. Appendectomy with abscess drainage was performed 3 days after admission. The mesoappendix was swollen with the abscess cavity and adhesion to the thickening wall of the cecum was severe. The appendix including the abscess in the mesoappendix was removed. Microscopic examination of the appendiceal wall showed severe inflammation with aggregation of neutrophil and lymphoid cells around the abscess cavity. Mucosa of the appendix showed mild inflammation. Pathological diagnosis was penetration of the cecal diverticulitis to the mesoappendix with abscess formation. The postoperative course was uneventful. The patient has been symptom-free for 15 months after the operation. Appendicular involvement with colonic diverticulitis is very rare, and only 2 cases have been reported.