A 61-yr-old man underwent Billroth I gastrectomy for an advanced cancer in the corpus of the stomach. On the first postoperative day, fresh bile discharged from the penrose drains, which had been placed in Winslow's foramen, and the volume of bile discharge subsequently increased. Leakage from the gastroduodenal anastomosis was excluded by gastroduodenography. Exploratory relaparotomy showed bile peritonitis with much more bile retention in the left subphrenic space. The origin of bile leakage could not be traced despite close examination of all the extrahepatic biliary tract and the liver surface. Intraoperative cholangiography through the cystic duct after cholecystectomy revealed that the bile leakage originated from an aberrant bile duct present in the free edge of the left triangular ligament (appendix fibrosa hepatis), which had been unintentionally cut at the primary operation. Knowledge of this anatomical structure is important and proper ligation is recommended when dissecting the appendix fibrosa hepatis to avoid postoperative bile peritonitis and the need for a relaparotomy.