A 78-year-old woman presented with preauricular superficial temporal artery (STA) aneurysm and scalp porocarcinoma, which had both increased in size over 2 years. She had no previous history of head trauma. Three-dimensional (3D) computed tomography (CT) angiography revealed a 4-cm diameter STA aneurysm arising from the main trunk of the left STA and located just lateral to the zygomatic arch. The scalp porocarcinoma was excised by dermatologists. The STA aneurysm was carefully dissected from the surrounding tissues, and was resected after ligation of the proximal STA. Histological examination showed the aneurysm consisted of intima, media, and adventitia, and the diagnosis was atherosclerotic fusiform aneurysm. 3D CT angiography is quite useful to plan surgical strategy for such an unusually large STA aneurysm.