Endotracheal tube obstruction causes serious complications, including cardiovascular instability, pneumothorax, pulmonary edema and brain death. A 64 year old man was scheduled for a laminectomy and instrument fixation due to spinal stenosis. The patient was intubated with a 8.5 mm reinforced endotracheal tube and turned to the prone position. An hour later, signs of partial endotracheal obstruction were observed including high airway pressure and low tidal volume. Airway obstruction signs were aggravated as the operation proceeded. Two hour later, passage of a suction catheter was difficult and PaCO2 increased significantly, so we temporarily stopped the operation and turned the patient to supine. After that, we exchanged the tube with another tube and found the distal tip of the reinforced tube impacted with mucous secretions.