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, one-and 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.