Primary aortoenteric fistula (PAEF) is a rare disease with a high mortality rate due to massive hemorrhaging and diagnostic difficulties. Although hemorrhagic regions can be identified by endoscopy, it is difficult to diagnose PAEF by this method. If PAEF is suspected, endoscopic procedure should be terminated and abdominal CT should be performed. Overlooking the herald bleeding of PAEF can lead to massive bleeding and death. An 85-year-old previously healthy male presented with a complaint of melena. Gastrointestinal endoscopy identified a hemorrhagic site in the third portion of the duodenum and endoscopic hemostasis was performed. However, during the procedure, it became apparent that the hemorrhage was probably not the result of a simple duodenal ulceration and abdominal CT was performed immediately. An aortic aneurysm connected to the duodenum was identified, confirming the diagnosis of PAEF. However, the patient died of massive hemorrhaging before an operation could be performed.