A 42-year-old male patient with obstructive sleep apnea syndrome (OSAS) suffering from biventricular heart failure is reported. He had been treated for OSAS with conventional therapy. However, he complained of severe dyspnea in association with extreme weight gain and general edema. Therefore, he was admitted to our department. He weighed 168 kg on admission, and marked edema was observed. Chest film revealed significant dilatation of the cardiac silhouette and pleural effusion. PaO2 was 37 mmHg and PaCO2 was 66 mmHg. Polysomnography showed an apnea index of 58.3 and severe oxygen desaturation during sleep. Right heart catheterization showed elevation of mean pulmonary artery pressure mPAP: 55 mmHg) and pulmonary capillary wedge pressure (Pcwp: 33 mmHg) suggesting biventricular heart failure. Digitalization and diuretic therapy were immediately initiated. In addition, nasal CPAP was applied to this patient during sleep, and sleep apnea and oxygen desaturation were almost completely reversed. Significant diuresis was observed, and blood gas data and sleep disturbance were improved. Fifty-nine days after admission, his weight had decreased to 96 kg, and mPAP and Pcwp decreased to 32 and 23 mmHg, respectively. This case demonstrates that nasal CPA is an effective tool for the treatment severe OSAS patients.