We report a simultaneously operated case of a 68-year-old man with lung cancer accompanied with aortic arch aneurysm. Preoperative staging CT for lung cancer incidentally demonstrated another lesion in the para-aortic arch area, which was suspected to be rather a pleural or intrapulmonary lesion by enhanced CT and MRI. However, this lesion was intraoperatively diagnosed as a cystic small sized-aneurysm. After a left upper lobectomy with lymph node dissection for lung adenocarcinoma (T2N0) was performed, this aneurysm was tightly wrapped using PTFE felt during the course of one operation. The difficulty of peroperative diagnosis by CT and MRI for small sized-aneurysm is discussed, and surgical stragety for lung cancer accompanied with such an aortic aneurysm is also commented.