Anomalous pulmonary venous connection is a relatively common associated anomaly in patients with atrial septal defect (ASD), particularly among those with the sinus venosus type. The incidence of partial anomalous pulmonary venous connection (PAPVC) is higher than 0.7% in the general population and 10% in patients with ASD. In this study, we present two cases with initial impression of ASD, the sinus venosus type in one and the secundum type in the other. The one with the sinus venosus type was found to have a PAPVC that drained into SVC, and the other was suspected of having the same problem because an abnormal shunt was found during cardiac catheterization. This speculation could not be proved, however, due to transesophageal echocardiogram failure. Because we feared the possibility of cardiac defects other than ASD, we performed a minimally invasive operation using a small midline incision instead of the submammary incision and did a full median sternotomy on the patient to look for other complicating coexistent cardiac defects. This patient and the former one were both proven intraoperatively to have a PAPVC that drained into SVC with sinus venosus ASD. The operation to correct an ASD is a rudimentary procedure, and it often becomes a common type of minimally invasive operation among young cardiac surgeons with limited experience. A submammary incision under the impression of simple ASD may meet with certain complications. Therefore, after our experience with the latter case, we do the minimally invasive operation using a small midline incision, which can be easily extended if need be.