A colonoscopic examination of a 58-year-old man revealed a small elevated lesion inside the orifice of the appendix. Using a polypectomy snare, a nodular polypoid lesion with a diameter of ca. 20 mm was removed from the lumen of the appendix. Histopathology showed that it was a well-differentiated adenocarcinoma. This is the first report of an intramucosal adenocarcinoma of the appendix diagnosed preoperatively; laparoscopy-assisted colectomy with a D2 lymph-node dissection was carried out. Endoscopists should consider an appendiceal tumor when an erosion, elevation, or deformity is seen in the head of the cecum.