A woman in her 80s was diagnosed with an abdominal mass during physical examination. Contrast-enhanced computed tomography(CT)revealed a tumor with contrast enhancement outside the ileocecal region of the intestine, and the ileocolic artery penetrated the tumor. No tumor was detected by colonoscopy. An endoscope could not be passed through due to an ileocecal valve stenosis. A biopsy of the ileocecal valve revealed only lymphocyte hyperplasia without adenocarcinoma components. Barium enema examination demonstrated no influx of the contrast medium from the cecum into the oral side of the intestine. Since a gastrointestinal stromal tumor in the ileocecal region was suspected, laparotomy was performed in the ileocecal region owing to the preoperative diagnosis of suspected malignant lymphoma, revealing a 5-cm elastic hard tumor outside the ileocecal wall. The tumor could not be separated from the intestinal tract. Histopathological examination revealed no lesion on the mucosal surface, although poorly differentiated adenocarcinoma infiltrated from the submucosa to the serosa. Thus, the patient was diagnosed with extramural growth-type ileocecal colon cancer. This disease is relatively rare but need to be kept in mind.