Acute airway obstruction during endotracheal anesthesia is embarrassing and critical situation which requires early diagnosis and immediate management. A 57-year-old man was scheduled for right pneumonectomy for a destroyed lung by fibroatelectatic changes and pleural calcification of right lung. We experienced high arterial PCO2 and inspiratory resistance during Left - Sided Double Lumen Endobronchial anesthesia in the left decubitus position. We exchanged tube after failure of suction and found airway obstruction due to mucous plug attached to the bevel of the endobronchial lumen.