Takayasu’s arteritis (TA) is an inflammatory disease of unknown etiology involving the aorta and its branches, and also causes aortic regurgitation (AR). One of the most serious but rare complications after aortic valve replacement (AVR) in TA is aneurysm formation of the sinus of Valsalva. A 64-year-old woman had undergone AVR with a prosthetic valve for AR due to TA 4 years earlier and had received an implanted permanent pacemaker for complete atrioventricular block (AVB) 2 years later. Aortography 4 years postoperatively demonstrated aneurysm formation (47 mm in diameter) at the sinus of Valsalva although preoperative aortography showed severe AR without dilatation of the sinus of Valsalva. We recommended reoperation for the aneurysm but the patient refused. The perioperative histopathological examination revealed extensive destruction of the medial elastic fibers. Both the fragility of the sinus of Valsalva and the residual inflammation could have caused the patient’s aneurysm formation. Moreover, extension of TA into the ventricular septum or mechanical compression of the aneurysm against the conduction system might have caused her progressive AVB. Close and lifelong follow-up for patients with TA regarding development of aneurysm after surgical treatment is indispensable when fragility of the aortic root had been confirmed.