A 73-year old woman consulted our hospital for consciousness disorder and was hospitalized under a diagnosis of cerebral infarction. On admission, retention of a large volume of ascites was noted. There were also marked increases in tumor markers. Serum CEA, CA19-9 and CA125 levels were 871.9 ng/ml, 1048.2U/ml, and 444.7U/ml, respectively. Cytological examination of the ascites showed class V. Abdominal CT showed a mass measuring 4 cm in diameter and dilatation of the small intestinal tract. During detailed examination, she developed ileus. Fluoroscopy through the lung tube suggested a small intestinal tumor. Laparotomy was performed to relieve ileus. In the abdominal cavity, the greater omentum formed a mass due to dissemination. Tumor was found in the jejunum approximately 60 cm from Treitz's ligament toward the anal side. Omentectomy and partial jejunectomy were performed. Macroscopic findings showed tumor protruding from the mucosal surface. Histological diagnosis was mucinous adenocarcinoma. Primary small intestinal cancer is very rare. In particular, only one case of mucinous adenocarcinoma of the small intestine was reported in Japan. We reported the second case in Japan with a review of the literature.