Tension pneumothorax during one-lung ventilation (OLV) is a rare but life-threatening complication. A 79-year-old male patient who was diagnosed with lung cancer underwent Univent(R) Tube (Fuji Systems Corporation, Tokyo) intubation for left upper lobectomy. Two hours after the initiation of OLV, the patient could not tolerate it. Thus, oneand two-lung ventilation were alternatively applied to continue the operation. After the operation, an emergent chest radiograph was taken, and pneumothorax was found at the right (dependent) lung field.