Obstructive sleep apnea (OSA) is characterized by recurrent upper airway collapse associated with arousal and/or oxygen desaturation. Although OSA is primarily a chronic illness, patients' condition may deteriorate to critical owing to acute cardiopulmonary failure. A 53-year-old man without specific medical history except OSA experienced a critical oxygen desaturation during diagnostic polysomnography accidently. He was exhausted by stress and heavy consumption of alcohol in those days. Untreated OSA can cause respiratory failure when sudden cardiopulmonary failure occurs during sleep, even if patients have no specific medical history.