Giant sinus node artery aneurysms with a fistula into the left atrium are extremely rare. A 76-year old woman was admitted to our hospital for an abnormality on her chest X-ray. Coronary computed tomography (CT) angiogram revealed a giant coronary aneurysm with feeding arteries arising from the sinus node artery and the left circumflex artery. Additionally, the aneurysm had a fistula into the left atrium. The size of aneurysm was 70 × 50 mm. Coronary angiography showed contrast dye drained from the aneurysm to the left atrium through the fistula. Surgical treatment was performed. After feeding arteries were ligated, the aneurysm was opened under routine cardiopulmonary bypass. The diameter of the fistula entry was found to be 2 cm, and it was closed using a patch material. Since we were not able to identify the anatomical relationship of the fistula, mitral valve and pulmonary veins through the origin of the fistula, we used an additional right lateral left atriotomy approach. The aneurysmal wall was resected, and the cause of aneurysmal change was histopathologically diagnosed as atherosclerosis. The patient's postoperative course was uneventful, with a postoperative CT showing a complete exclusion of the aneurysm.