An 82-year-old man suffering from shortness of breath, leg edema, and appetite loss visited our outpatient clinic. Although he was diagnosed with atrial septal defect (ASD) requiring surgical repair 40 years ago, he had refused an operation at that time. Echocardiography revealed a 37-mm ASD and massive pericardial effusion. Cardiac catheterization showed significant left-to-right shunt flow with Qp/Qs of 4.6 and pulmonary artery pressure of 93/35/52 mmHg. Pulmonary vascular resistance was calculated as 8.3 Wood units. Surgical treatment was no longer indicated due to his condition, and percutaneous treatment with an Amplatzer septal occluder (ASO) was planned instead. Diuretics, a PDE3 inhibitor and nasal oxygen, were administered preoperatively for 1 month. A 38-mm ASO, the maximum occluder size available in Japan, was implanted successfully. No acute decompensation occurred after ASD closure, and the patient's symptoms improved after ASO implantation. Cardiac catheterization on postoperative day 13 revealed no evidence of residual shunt, and pulmonary artery pressure decreased to 63/20/33 mmHg. As postoperative therapy, a PDE5 inhibitor, endothelin receptor blocker, and PGI 2 analog were administered for residual pulmonary hypertension. Because pericardial effusion did not disappear after pericardiocentesis, surgical pericardiostomy was performed 6 months after ASD closure, which reduced PA pressure to 34/16/24. Appropriate pre-and postoperative medical therapy, device closure with an ASO, and pericardiostomy were effective in this frail patient with a giant ASD with pulmonary hypertension and massive pericardial effusion.