A 52-year-old man had a history that included aortic valve replacement due to infectious endocarditis in 1987. Chest X-ray showed slight enlargement of the superior mediastinum in 1998, but the enlargement was very mild and there had not been any significant change since 1998. However, chest X-ray demonstrated an extremely protruding mass on the right side of the superior mediastinum in May 2004 and a pseudoaneurysm located in the ascending aorta was demonstrated by computed tomography. We considered this aneurysm had been caused by ascending aortic cannulation for blood return from cardiopulmonary bypass (CPB) during the previous surgery. On re-operation, CPB was established by femoro-femoral bypass and median sternotomy was performed. The pseudoaneurysm measured 60mm in diameter and there was a felt-pledget on top of the aneurysm. Under deep hypothermic cardiac arrest, we incised the aneurysm and closed the orifice of the pseudoaneurysm using a patch (Hemashield Woven Fabrics). On pathological examination, the wall of the pseudoaneurysm showed a structural loss of the blood vessel and the felt-pledget had been exposed to the inferior of the aneurysm breaking through the wall. We considered this a non-mycotic pseudoaneurysm because of this patient's clinical course, surgical and pathological findings. We encountered a pseudoaneurysm in the ascending aorta that was detected and treated surgically about 20 years after aortic valve replacement.