The case was a 70-year-old female. Atrioventricular septal defect was diagnosed in her childhood, however, surgical treatment had not been performed. The patient had suffered from heart failure at the age of 69, and she was referred to our hospital for treatment. Her diagnosis was intermediate type atrioventricular septal defect, moderate left atrioventricular valve regurgitation, membranous ventricular septal aneurysm and atrial flutter. An autologous pericardial patch was used to close the ostium primum type atrial septal defect associated with simultaneous covering of membranous ventricular septal aneurysmal wall. Concomitant left and right atrioventricular valvuloplasty and arrhythmia surgery were performed. Her postoperative course was uneventful and the patient was discharged from our department on the 16th postoperative day. To our knowledge, there are few reports of surgery for incomplete type atrioventricular septal defect in the elderly and no report for intermediate type atrioventricular septal defect in Japan. In incomplete type atrioventricular septal defect, symptoms such as supraventricular arrhythmia and heart failure develop according to aging. Reported surgical results in the elderly are quite good, and improvement of excise tolerance is expected. Precise evaluation and proper indication of surgical treatment is mandatory even in older patients.