An 85-year-old woman had a history of coronary artery bypass grafting (CABG) performed 7 years ago, and dyspnea on effort had been worsening recently. Since echocardiography showed severe mitral valve regurgitation( MR), mitral valve repair was suggested. Preoperative enhanced computed tomography (CT) showed the patent functioning left internal thoracic artery (LITA) graft. Mitral valve replacement (MVR) using a 25 mm CEP bioprosthesis was performed successfully via resternotomy without any intraoperative injury of the heart. Myocardial protection without clamping of functioning LITA was done by both antegrade and retrograde continuous coronary perfusion (RCCP) under mild hypothermia. The postoperative clinical course was uneventful without any hemodynamic compromise. She was discharged on postoperative day 21 without any cardiac events following early introduction of cardiac rehabilitation. From these results, mitral valve reoperation by RCCP under mild hypothermia without control of functioning internal thoracic artery( ITA) grafts could be a safe option in some cases.