A case of the incomplete double aortic arch was observed on a male Japanese subject (69 years old) in students' dissection of Hokkaido University School of Medicine. The ascending aorta was directly followed by the right aortic arch, from which the left brachiocephalic trunk, right common carotid artery and right subclavian artery successively arose. As the cord-like structure (8 mm in length, 2 mm in diameter) of the left aortic arch was found between the left subclavian artery and descending aorta, the left brachiocephalic trunk seemed to be essentially equivalent to the initial portion of the left aortic arch. Thus, this anomaly of the aortic system was concluded as the incomplete double aortic arch. Through the vascular ring formed by the cord-like left aortic arch and right aortic arch, the trachea and esophagus passed. So-called aortic diverticulum was found at the site where this cord-like structure attached to the descending aorta. Histological examinations of the cord-like left aortic arch demonstrated that (a) it was completely closed and any structure suggesting to the tunica intima could not be also identified; (b) elastic and collagen fibers were scattered within the stroma of this structure, and somewhat concentrically arranged at its peripheral margin; and (c) several small blood vessels penetrated into this structure. The ligamentum arteriosum normally arose from the left pulmonary trunk, and connected with the aortic diverticulum just described above. The left and right recurrent laryngeal nerves respectively looped under the ligamentum arteriosum and right aortic arch. The thoracic duct did not open into the left venous angle, but into the right one.