A 94-year-old woman was admitted to our hospital with sudden onset of chest pain without any episode of trauma. Computed tomography(CT) revealed left massive pleural effusion and extravasation of contrast medium from the side chest wall to the back. Neither aortic aneurysm nor dissection was evident. During the investigation, the patient went into hypovolemic shock. Hemothorax due to bleeding from an intercostal artery was diagnosed, and emergency surgery was performed. A 1-mm hole was detected in the descending aorta, and closed by a single suture. The final diagnosis was spontaneous rupture of the thoracic aorta. Three-dimensional CT (3D-CT), reconstructed postoperatively, revealed extravasation of the contrast medium from the descending aorta. The postoperative course was satisfactory, and the patient was discharged on the 20th postoperative day. 3D-CT may be useful for identifying the source of bleeding in such cases.