We have experienced a case of coronary sinus atrial septal defect (ASD) with ventricular septal defect (VSD). Cardiac catheterization revealed a step-up of oxygen saturation in right ventricle, L-R shunt 70.3%, R-L shunt 6.9% and pulmonary hypertension (PH). Since diagnosis of coronary sinus ASD was difficult, the operation was performed under diagnosis of VSD. Although the operation was success, cyanosis appeared after the operation. Contrast echocardiography using subcostal 4 chamber view showed coronary sinus ASD with a large Thebesian valve and the right-to-left shunt of the ASD. The Thebesian valve seemed to lead the venous return flow to the left atrium. Coronary sinus ASD was closed using a Xenomedica patch.