Left main coronary artery atresia is an extremely rare disease. Differential diagnosis of left main coronary artery atresia from atherosclerotic occlusion of left main coronary artery is difficult even if performing invasive coronary angiography. We present a case of a 48-year-old male with left main coronary artery atresia. Echocardiography showed left ventricular dysfunction. Invasive coronary angiography showed absence of left main coronary artery. A multidetector computed tomography showed a string-like structure at the site of left main coronary artery. A diagnosis of left main coronary artery atresia was made, and he underwent coronary artery bypass grafting. At the operation, a thin and not-sclerotic left main coronary artery was observed. Echocardiography, performed after the surgery, showed normalization of the left ventricular systolic function. Multidetector computed tomography might be a useful method to diagnose left main coronary artery atresia.