Duodenal metastasis from primary lung cancer is extremely rare. It rarely shows any symptoms, and the prognosis for this condition is poor. We herein describe the case of a 46-year-old woman with primary lung cancer who underwent a left upper lobectomy. Severe anemia was observed about 20 days after lobectomy. Gastroduodenoscopy showed duodenal metastasis. Simultaneously, brain metastasis was also detected using magnetic resonance imaging. The patient underwent a local resection of the duodenum and a tumor resection of the brain. Postoperative irradiation of the brain metastases and systemic chemotherapy of the lung metastases were performed, and complete remission occurred. However, abdominal lymph node metastasis recurred, and the patient died 1 year after the lobectomy.