The internal mammary artery has become the primary conduit for the surgical revascularisation of the left anterior descending artery. Large side branches of internal mammary artery are typically ligated during cardiac surgery to avoid a potential coronary steal phenomenon. However, ligation of side branches can be unsuccessful due to the technically difficult surgical exploration of internal mammary artery branches. In this article, we present a case of a man who suffered from exertion angina pectoris despite successful surgical revascularisation of occluded left anterior descending artery by the internal mammary artery bypass. The coronary steal syndrome caused by the mighty side branch of internal mammary artery, that is, lateral costal artery was concluded as the reason of exertional angina. The endovascular procedure was performed and the lateral costal artery was successfully occluded using vascular plug. The occlusion of lateral costal artery has led to a complete disappearance of the exertional angina.