A 73-year-old male was admitted to Tokyo University Hospital due to cardiac arrest secondary to ventricular arrhythmias. Although the patient survived after cardiopulmonary resuscitation, he needed mechanical ventilation. When we tried to wean the patient from mechanical ventilation, we found that he suffered from frequent sleep apneas of over 40 times/hour. To evaluate the severity and frequency of the apneas, we performed polysomnographic study on the patient under assist ventilation. The polysomnographic study revealed that frequency of apneas was very high but the magnitude of desaturation was not so severe during the night. Since we confirmed the lowest SaO2 in this patients was over 90% during day and night, we tried to wean from the patient from the ventilator again and succeeded. The clinical application and usefulness of nocturnal ventilatory monitoring for patients with mechanical ventilation was discussed.