A 34-year-old woman was referred to our hospital for inspection before operation for liver transplantation as a donor for her son. She had no symptoms and remarkable medical history. Echocardiography showed a defect in lower part of the atrial septum with a cleft of left atrioventricular valve and right atrioventricular valve regurgitation. She was diagnosed as incomplete atrioventricular septal defect for the first time. She underwent double valve repair and patch closure of the atrioventricular septal defect. It is a rare case for atrioventricular septal defect in adulthood incidentally detected without symptoms.