Tricuspid atresia with unroofed coronary sinus is an extremely rare cardiac anomaly. A 15-year-old boy who was successfully operated was suspected this disease preoperatively. We speculated this disease by right atrial angiography and underwent the direct suture closure of a coronary sinus defect and Fontan's operation. When we perform Fontan's operation on tricuspid atresia, we should take the existence of unroofed coronary sinus into our consideration. The echocardiography, cardiac catheterization and angiography, especially, right atrial angiography had to be carried out preoperatively. During the operation, it is important to check over the influence of cardioplegic solution from the ostium of coronary sinus. If not, elaborate inspection have to be done on the back wall of the left atrium through ASD to search for abnormal communication directly or by inserting sound from coronary sinus.