A 69 year old man who was admitted with hoarseness and diagnosed as aneurysm of the diverticulum of the ductus arteriosus was reported. Operation was performed through a median sternotomy under partial cardiopulmonary bypass. Saccular form aneurysm, had a stalk attaching to left pulmonary artery, was repaired using Dacron patch prosthesis. His postoperative course was uneventful except transient left pleural effusion. Because of fragirity of aneurysm in the adult, early surgical intervention is recommended. To our knowledge, this is 11th surgically treated case to be reported in the literature in Japan.