CASE PRESENTATION: A 52-year-old Japanease man presented with a 1-month history of fever, general malaise, edema of legs and low back pain. He had no significant medical history. We performed transthoracic echocardiography, and detected sessile vegetations, below 10mm in size, on the posterior mitral valve leaflets (P2) and anterior mitral valve leaflet (A2). In addition, echocardiography showed a mitral severe regurgitation caused by the posterior mitral valve leaflet (P2) prolapse. A definitive blood culture revealed the presence of streptococci mutans. Based on the above findings, we diagnosed infective endocarditis with relatively large sessile vegetation. The patient was started on a 2-week course of intravenous antibiotics, benzylpenicillin + gentamicin after Sulbactam / Ampicillin + gentamicin. But clinical response was not good, persistent sessile vegetation and constant high C-reactive protein level. Thereby, we planned mitral valve operation. Surgery was performed with cardiopulmonary bypass and mild hypothermia after achievement of general anesthesia. There was no sessile vegetation that pointed by preoperative echocardiography, but mitral valve leaflets were billowing with excess tissue. In addition, there were granular vegetations on bi-mitral valve leaflets, and P2 chordae was ruptured. Hence, infected bi-Mitral valve were resected, we operated mitral valve replacement by a mechanical valve. Ampicillin was administered for 6-weeks after surgery. Control of infection after operation was satisfactory. The patient discharged hospital on 42-days after surgery.