A 65-year-old woman who had been diagnosed with a thoracic aneurysm was admitted to our hospital because of loss of consciousness. Brain CT revealed that the left corticomedullary junction is obscured. Contrast-enhanced CT demonstrated an acute type A aortic dissection with right internal carotid artery occlusion, left internal carotid artery stenosis, and severe pectus excavatum. Although the consciousness level at the time of admission was JCS200, it gradually improved and she regained spontaneous movement of the right side of her body. Repair of the acute type A dissection was indicated because her neurological deficit had improved. The surgery was performed via an L-shaped approach consisting of a median sternotomy and a left 5th intercostal thoracotomy with moderate hypothermic circulatory arrest and selective cerebral perfusion. An entry was found in the aortic arch between the origins of the brachiocephalic artery and the left common carotid artery, and a partial arch replacement was performed using a four-branched artificial graft. Although the right hemiparesis remained, she recovered well and was transferred to a rehabilitation hospital at 45 days postoperatively. The L-incision approach obtained a good surgical field in a patient with a type A dissection and severe pectus excavatum.