A 49-year-old man was admitted to hospital for investigation of a mediastinal shadow seen on a chest radiograph. Chest completed tomography revealed a mediastinal mass of 65 x 55mm. At surgery, the mass was found to be contained within the upper mediastinum and adherent to the vertebrae, esophagus, trachea, and superior vena cava. We therefore selected sequential approaches using a lateral incision for the thoracotomy and a modified transmanubrial approach. The lateral incision enabled detachment of the adhesion between the mass and the posterior to median mediastinum, and the modified transmanubrial approach was useful for separating the mass from the upper to anterior mediastinum. The mass had no connection to the cervical thyroid gland. Histological examination revealed a large mediastinal cyst of an ectopic thyroid with small nodules diagnosed as papillary carcinoma. There was no recurrence 14 months after surgery.