A life-long anticoagulation for the prevention of ischemic cerebral events by thromboemboli has been conventionally introduced for the treatment of an atrial septal aneurysm. However, due to the recent decrease of the risks in the open heart surgery, the alternative therapeutic modality has been introduced for the prevention of complications of the anticoagulation. A 41-year old female with dizziness was admitted to our hospital. She was diagnosed a shaving atrial fibrillation and a cerebellar infarction, and an atrial septal aneurysm was detected by transesophagel echocardiography that was not detected by the transthoracic echocardiography. Surgery was approached to the right submammarial anterolateral thoracotomy. The atrial septal aneurysm was obliterated by a purse-string suture and plication. Surgial results were excellent with normal sinus rhythms and esthetically satisfying appearance.