Lucas-Schmidt IIA type of Cor triatriatum has been rarely reported. Since it shows the same hemodynamics as Darling IIb type TAPVC, it is important as a emergent surgical case in early infancy. We reported a male neonate with IIA type Cor triatriatum. He was admitted to our hospital because of cyanosis and dyspnea since birth. The echocardiographic examinations revealed the accessory chamber behind the left atrium and atrial septal defect. The view from the right atrium revealed that there was no direct connection between the accessory chamber and the true left atrium, and we diagnosed it Lucas IIA type Cor triatriatum. The abnormal diaphragma was resected, and the atrial septum was corrected with a Xenomedica patch. The patient had an uneventful postoperative course, and was discharged on the 25th postoperative day.