A case was 6-year-old female diagnosed as the cor triatriatum. The magnetic resonance imaging synchronized at R wave on the electrocardiogram was performed in order to see the abnormal septum in the left atrium. In the horizontal view, the abnormal septum that divides the left atrium into the accessory left atrial chamber and the true left atrium, was well visualized. This septum lied on the vertical portion. Operation was carried out with the approach through the accessory chamber and the fibromuscular membrane were excised. Preoperative imaging of the MRI was useful and helpful for us to reach to a decision of the surgical approach in this patient.