A 62-year-old man was referred to our hospital because of abdominal pain. Computed tomography revealed an approximately 7-cm-diameter tumor in the left abdomen with metastatic lymph nodes, an approximately 1-cm-diameter round tumor in contact with the subclavian artery in the apical lobe of the right lung, and mediastinal lymph node enlargement in contact with the superior vena cava. Esophagogastroduodenoscopy and colonoscopy revealed no abnormalities. Double-balloon endoscopy revealed a whole circumferential ulcer in the jejunum approximately 20 cm from the ligament of Treitz. Biopsy analysis of an ulcer specimen revealed a poorly differentiated carcinoma. Immunohistochemical staining of the specimen showed that it was positive for thyroid transcription factor 1 and cytokeratin 7 and negative for cytokeratin 20, GATA-binding protein 3, caudal-type homeobox protein 2, and paired box 8. Positron emission tomography revealed positive findings in the small-intestinal tumor, nearby mesenteric lymph nodes, lymph nodes around the abdominal aorta, lung tumor, and mediastinal lymph node in the apical lobe of the right lung. Accordingly, the patient was diagnosed as having a lung carcinoma with small-intestinal metastasis (T1b, N3, M1c; cStage IVB). Pathological examination helped distinguish the primary small-intestinal tumor from the metastatic small-intestinal tumor and detect the tumor origin.