Robotic surgery with carbon dioxide (CO2) insufflation to the thorax is frequently performed to gain a better operative field of view, although its intraoperative complications have not yet been discussed in detail. We treated two patients with difficult ventilation caused by distal migration of a double-lumen endotracheal tube (DLT) during robotic thymectomy. In the first case, migration of the DLT during one-lung ventilation (OLV) occurred after CO2 insufflation to the bilateral thoraxes was started. Oxygenation rapidly deteriorated because dependent lung expansion was restricted by CO2 insufflation. In the second case, migration of the DLT during OLV occurred while CO2 insufflation to a unilateral thorax and mediastinum was performed. In both cases, once migration of the DLT during OLV occurred with CO2 insufflation, readjusting the DLT became very difficult because our manipulation of bronchofiberscopy was prevented by the robot arms located above the patient's head and because deformation of the trachea/bronchus induced by CO2 insufflation caused a poor image of the bronchofiberscopic view. Thus, during robotic-assisted thoracoscopic surgery with CO2 insufflation, since there is a potential risk of difficult ventilation with a DLT and since readjustment of the DLT is very difficult, discontinuing CO2 insufflation and switching to double-lung ventilation are needed in such a situation.