A 61-year old man was referred to our institute under a diagnosis of pulmonary aneurysm. Contrast computed tomography revealed a huge pulmonary aneurysm of 70 mm in maximal dimension at the main pulmonary trunk. No congenital heart disorders were identified on trans-thoracic or trans-esophageal echocardiography. No significant signs of pulmonary hypertension were demonstrated on right heart catheterization. Laboratory findings on admission included positive results for syphilitic antibodies. T-shaped graft replacement of the pulmonary arteries using a cardiopulmonary bypass was scheduled. The main and left pulmonary arteries were replaced with a J-Graft 26 mm in size (Japan Lifeline, Tokyo, Japan). Then, the right pulmonary artery was reconstructed with the rest of the J-Graft, and anastomosed to the side of the newly reconstructed main and left pulmonary arteries. His postoperative course was generally uneventful. Pathological findings of the excised aneurysmal walls revealed true aneurysmal formation with no specific inflammatory changes. This case was considered to be an idiopathic pulmonary aneurysm without congenital heart disorders, pulmonary hypertension, and pathologically inflammatory reactions of aneurysmal walls.