The mitral-aortic intervalvular fibrosa (MAIF) is a fibrous skeleton between the mitral and aortic valves. We describe a case of a pseudoaneurysm of MAIF (MAIFPsA) 3 months after an aortic valve replacement. A 75-year-old man had undergone aortic valve replacement using a bioprosthesis (SJM Trifecta 23 mm). He developed congestive heart failure 3 months after surgery. Trans-esophageal echocardiography and multi-detector computed tomography detected a MAIFPsA and severe paravalvular leakage. The patient underwent a new operation. After the aortic prosthesis was removed, an orifice of the pseudoaneurysm, which was composed of the anterior mitral leaflet and the aortic annulus, were revealed. The defect was repaired with a bovine pericardial patch, sutured to the anterior mitral leaflet and aortic annulus. A new bioprosthesis (Carpentier-Edwards PERIMOUNT 21 mm) was implanted in the supra-annular position. His postoperative course was uneventful.