A 73-year-old man with a history of tarry stools was admitted to our hospital. Gastroduodenoscopy on admission revealed an ulcer with an exposed vessel at the duodenal bulb. During endoscopic clipping for prevention of re-bleeding, a large amount of bleeding from the vessel was observed. Hemostasis was achieved after the procedure. However, 7 days later, the patient passed a large amount of tarry stools and experienced hypovolemic shock. Endoscopic re-examination revealed a pulsatile aneurysmal vessel at the base of the duodenal ulcer. Abdominal contrast-enhanced computed tomography and abdominal angiography demonstrated an aneurysm of the gastroduodenal artery, which was successfully treated with transcatheter arterial embolization. Thus, here we report a rare case of an aneurysm of the gastroduodenal artery caused by a duodenal ulcer.