We present an unusual case of a ruptured descending thoracic aortic aneurysm into the right pleural cavity of a patient with pectus carinatum. The presence of pectus carinatum played an important role in the development of the aneurysm at the atypical site and the rupture into the right pleural cavity. A small amount of right pleural bleeding on admission can increase and develop to massive hemothorax until emergency operation. Massive bleeding in the right pleural cavity where the dependent lung is located causes atelectasis and increased shunt fraction under one lung ventilation. Therefore, continuous drainage of the right pleural cavity is essential to prevent serious hypoxia during graft replacement in a case of ruptured descending thoracic aneurysm into the right hemithorax.