A 52-year-old man, presenting with exertional chest pain was investigated to explain his chest pain. Echocardiographic examination revealed he had the ostium secundum type of atrial septal defect, dilated right heart chambers, dilated pulmonary artery, and pulmonary artery systolic pressure of 65 mm Hg calculated from tricuspid regurgitation, but his chest pain could not be explained with these findings. Therefore, cardiac catheterization and coronary angiograph were performed. Coronary angiography revealed severe stenosis of the left main coronary artery and otherwise normal vessels. Angiographic images made us think that there might be an external compressing structure on the left main coronary artery. For further evaluation, contrast-enhanced magnetic resonance images of the heart were taken. These images showed that the markedly dilated pulmonary artery was compressing the left main coronary artery.