The chest radiograph of a 57-year-old man, complaining of paroxysmal dyspnea, suggested the probably of a tumor. Chest CT showed a tumor containing calcification, behind the left crus of the diaphragm. Chest MRI suggested lipid components and a cystic lesion within the tumor. Their findings were clinically compatible with posterior mediastinal teratoma. The pathological diagnosis of the surgically resected tumor was mature teratoma with neither malignant components nor thymic tissue. Study of past case reports suggests that posterior mediastinal teratomas should have less malignant characteristics than anterior mediastainal teratomas. Our case is the fifteenth case report in the Japanese literature, and accumulation of more cases is required.