A 65-year-old woman was admitted to our hospital complaining of chest pain in 2012. Coronary computed tomography angiogram revealed left main coronary artery atresia (LMCAA) with congenital absence of the left main trunk, and LAD received several collaterals perfusing from the RCA. She was observed because her symptoms were transient. However, her dyspnea on effort gradually became worse in 2016. Treadmill exercise electrocardiography showed ST depression in II・III・aVF and V4-6. Also, echocardiography showed severe MR due to the prolapse of the anterior leaflet and moderate TR. We performed mitral valve plasty and tricuspid valve plasty, and coronary artery bypass grafting using the left internal thoracic artery to the left descending artery in March, 2017. The postoperative course was uneventful. She was discharged in good condition 14 days after operation. LMCAA is a rare coronary anomaly. There are only 53 published case reports, and a few surgical reports in adult cases. This is a rare adult case of surgical treatment of LMCAA.