To our knowledge, only five cases of malignant changes in small-bowel duplication have been reported previously in the English-language literature, and these were found only in ileal duplications. We describe a unique case of adenocarcinoma arising in a tubular duplication of the jejunum; this is the oldest patients in whom malignant tumor of alimentary tract duplication has been reported. A 73-year-old man was admitted to our hospital because of lower abdominal pain. Computed tomography (CT) and ultrasonography showed two abdominal masses; one, about 5 cm in size, was observed in the upper abdomen, the superior mesenteric artery (SMA) and the other, which was a well-demarcated heterogeneous mass, was revealed in the lower abdomen. Laparotomy revealed the mass in the upper abdomen involved the SMA and transverse colon, while the mass in the lower abdomen was of white globular form, ping-pong ball size, in the jejunal serosa approximately 40 cm on the anal side from Treiz's ligament. Transverse colostomy and a partial jejunectomy were performed, and the tumor was resected. Histologically, the diagnosis was adenocarcinoma arising in a tubular duplication of the jejunum. The patient died of necrosis and perforation of the intestine due to obstruction of the SMA 7 months after the surgery. Autopsy revealed that the upper abdominal mass consisted of metastatic foci associated with the lymph node of the SMA.