A 46-year-old woman was admitted to our hospital with abdominal discomfort. Chest computed tomography (CT) showed multiple nodules in both lung fields. Bronchofiberscopy was performed and adenocarcinoma was confirmed cytologically. Biopsy of a cervical lymph node revealed mucinous adenocarcinoma with signet-ring cells. No evidence of a primary tumor could be found elsewhere, despite examinations including CT scan, fluorodeoxyglucose-positron emission tomography, gastroscopy, colonoscopy and capsule endoscopy. The patient was therefore diagnosed with mucinous adenocarcinoma with signet-ring cells of unknown primary origin. First-line chemotherapy with TS-1/cisplatin was effective,but the patient died of respiratory failure 22 months after her first visit. Although the clinical presentation and radiographic findings were dominant in the lung, the primary tumor site remained unknown.