We report a case of unroofed coronary sinus not associated with the persistent left superior vena cava. Definite diagnosis of the unroofed coronary sinus was obtained by trans-oesophageal echocardiography, which revealed the unroofed portion with left-to-right shunt. Real-time three-dimensional trans-oesophageal echocardiography could show the whole pictures of the defect, which was useful information for surgical repair.