Among congenital dilatations of the common bile duct, a congenital diverticulum of the common bile duct may remain clinically latent for a long period. One case is reported in a 77 year old subject. Precise diagnosis of the disease is often made at operation and on pathological examination. The diverticulum is cured in most cases after surgery which consists of simple removal, but which may sometimes require reconstruction of the common bile duct.