We report a case of multiple colonic ulcers associated with cancer of the ascending colon, with difficulty in making the differential diagnosis. An 87-year-old woman was admitted to the hospital with a diagnosis of acute appendicitis and underwent an appendectomy. Since CEA and CA19-9 levels were found to be high at examination of blood on admission, close explorations of the patient's whole body were conducted few days after surgery and type 2 cancer of the ascending colon and multiple colonic ulcers were detected by a colonoscopy. So the patient was operated on again. Intraoperative examination revealed multiple ulcerated lesions in the right hemicolon as well as cancer of the ascending colon. Because we could not clearly rule out malignancy, intraoperative colonoscopy was performed to assess the remaining colonic ulcers. Finally, we decided to perform a wide right hemicolectomy. On histopathological examination, the type 2 lesion of the ascending colon was diagnosed as moderately differentiated adenocarcinoma. A total of 5 punched-out, deep, semicircular ulcers were observed on the anal side of the tumor, but were found to show no malignancy. The patient had no symptoms suggestive of Behçet's disease and the features of her multiple ulcers were compatible with a diagnosis of idiopathic colonic ulcer.