A 67-year-old woman was admitted for radiofrequency catheter ablation (RFCA) of paroxysmal atrial fibrillation. She had underwent patch closure of atrial septal defect 7 years before. Her arterial blood oxygen saturation level was 94.6% without any oxygen supplement. Her electrocardiogram, chest X-ray, and echocardiogram were within normal limits. She underwent cardiac computed tomography for the left atrial reconstruction before the RFCA. It revealed abnormal drainage of the middle cardiac vein into the left atrium (Fig. 1). The other cardiac veins including the great cardiac vein and small cardiac vein drained into the coronary sinus normally. While she underwent RFCA, the abnormal drainage of the middle cardiac vein was also confirmed in the CARTO® image (Fig. 2). An anomaly of the cardiac vein has rarely been reported. Previously reported venous anomaly of the heart was anomaly of the great cardiac vein (1) or totally abnormal drainage of the cardiac veins (2). This is a unique case of an isolated anomalous drainage of the middle cardiac vein into the LA without any anomaly of the coronary sinus.