A 8-month-old boy with ostium secundum atrial septal defect and persistent left superior vena cava draining into the coronary sinus underwent surgical treatment because he had required long-term ventilator support due to congestive heart failure and respiratory distress. The left superior vena cava and dilated coronary sinus produce a ridge in the left atrium, causing a subdivided left atrium. In this case, the early symptom could be explained by an obligatory left to right shunt at the level of the left atrium resulting from a ridge produced by the left superior vena cava and the colonary sinus, as a subdivided left atrium.