A 56-year-old man visited another hospital complaining of hemoptysis. A chest radiograph showed expansion of the left upper mediastinum which seemed to be a mass-like lesion. He was referred to our hospital for further investigations. Before further examination, however, he presented to the emergency room with sudden onset of severe back pain. Rupture of a thoracic aortic aneurysm was suspected because of the clinical symptoms and the findings of emergency enhanced CT scanning. Emergency surgery was performed at the other hospital, and frozen section results indicated that the lesion was a non-small cell lung cancer. The pathology report of the surgical specimens revealed poorly differentiated adenocarcinoma of the lung with infiltration of the aortic wall. Postoperative chemotherapy was added, and the patient is doing well 10 months after operation. Some cases of tumor mimicking aortic aneurysm have been reported. We reported this case of lung cancer mimicking the rupture of a thoracic aortic aneurysm.