Ventilation impairment sometimes developes in patients with a double-lumen tube (DLT) during thoracic anesthesia. A 27-year old man who suffered from a left pneumothorax showed ventilation impairment with right lung obstruction after the induction of anesthesia. We checked breathing sounds before and after clamping each lumen. Correct positioning was confirmed by direct visualization using a flexible fiberoptic bronchoscope and by chest X-ray. The DLT was removed and replaced by a single lumen endotracheal tube because of severe ventilation impairment. On careful examination of the DLT, manufacturing defects in a bifurcated connector were noted, and its openings were occluded with a thin plastic membrane. After replacing the DLT, the ventilation impairment was improved. It is important to test DLT prior to use, and to maintain an awareness that airway obstruction can occur due to structural defects.