A 59-year-old woman underwent explorative laparotomy under general anesthesia for mechanical ileus. The patient had rectal cancer with multiple metastasis, and was receiving 1.5-2 mg of intravenous morphine per hour due to severe abdominal pain. After about 3 hours of general anesthesia, the patient was extubated and transferred to postanesthesia care unit. The patient was supplied with 6 L/min of oxygen by facial mask. In 30 minutes, the patient showed no response to verbal order and pain stimulus with sluggish eye reflex, although pulse-oximeter showed 98-99%. After emergent intubation, arterial blood gas was sampled, and the result showed severe acidosis with hypercapnia. The patient was transferred to intensive care unit, and after 1 hour of mechanical ventilation the patient became conscious then fully recovered without further complication.