A successful surgical correction of an incomplete endocardial cushion defect (ECD) with an ostium primum defect in a 63-year-old man is reported. Incomplete ECD with ostium primum defect often causes severe heart failure in infancy. Reports of its surgical correction in elderly patients are few. The patient had upper abdominal discomfort and grade 1 mitral valve regurgitation. The preoperative diagnosis was complete ECD (Rastelli type A) because we misdiagnosed a leakage from a cleft between the left superior leaflet and the left inferior leaflet for a flow through a ventricular septal defect when we analyzed a preoperative left ventriculography. The importance of not misdiagnosing the leakage and echocardiography in preoperative diagnosis of ECD was therefore realized. The operative procedure involved patch closure of the ostium primum defect and mitral valve annuloplasty by Kay's procedure and the mitral valve regurgitation completely disappeared. The postoperative course was uneventful. The upper abdominal discomfort and cardiomegaly improved. If there is no severe dysfunction of other organs, surgical correction of incomplete ECD should be recommended even for elderly patients.