A 70-year-old non-obese man with no history of cardiopulmonary disease presented 4 times to the emergency room because of sudden onset of seizure during sleep. Each time he recovered within a few hours without any medication. Nocturnal polysomnographic recording revealed severe obstructive sleep apnea syndrome (OSAS, AHI 52.4/Hr). Nasal continuous positive airway pressure (n-CPAP) therapy was performed with 10cmH2O of pressure. His symptoms of severe daytime sleepiness and seizure were diminished. CPAP was decreased from 10cmH2O to 6 cmH2O later, because the patient complained with its high pressure. He then felt daytime sleepiness and suffered seizures during sleep again, and was re-admitted to our hospital. Chest roentgenogram taken at this admission showed remarkable pulmonary edema. We found that the pulmonary edema was recognized every time on his chest roentgenogram taken when he complained seizure. In addition, subsequesnt roentgenograms also showed that the pulmonary edema was diminished soon. On the other hand, his AHI was high (24.7/hr) even when he was under 6cmH2O of n-CPAP. We concluded that incompletely treated OSAS might lead not only to pulmonary edema, but also to seizures in this patient.