A 72-year-old woman was admitted with an abdominal tumor, which had been detected by computed tomography scanning. Endoscopic examination of the upper gastrointestinal tract revealed a type 3 tumor in the descending limb of the duodenum. A diagnosis of adenocarcinoma was made on the basis of the histological analysis of the biopsy sample. Pancreatoduodenectomy was performed, and peritoneal dissemination was confirmed at surgery. The pathological findings indicated primary duodenal cancer (mucinous carcinoma) associated with tumor thrombosis in the accessory pancreatic duct. Postoperative chemotherapy was performed. Carcinomatous peritonitis was controlled and the patient showed long-term survival.