A man in his 60s visited a local clinic because of repeated bouts of intermittent epigastric and back pain since July 2017. He was referred to our department because of suspected acute abdomen. Contrast-enhanced computed tomography revealed an aneurysm in the anterior inferior pancreaticoduodenal artery, and a retroperitoneal hematoma was observed. Although no extravascular leakage of contrast medium was observed, rupture of the aneurysm was suspected based on his vital signs, which indicated a state of shock. Emergency arteriography and coil embolization were performed. After coil embolization, the hematoma started to decrease, and no recurrent bleeding was observed. However, on hospitalization day 10, mucosal edema of the descending limb of the duodenum, thought to be an ischemic change, was observed along with gastrointestinal obstruction. A stomach tube was placed, and the patient was treated with central venous hyperalimentation for approximately 3 weeks. Because the gastrointestinal contrast radiography performed on hospitalization day 30 indicated improvement in the obstruction, liquid diet was started. Subsequently, the patient's gastrointestinal obstruction gradually improved. He was discharged on hospitalization day 47. The cause of the aneurysm in the anterior inferior pancreaticoduodenal artery was segmental arterial mediolysis (SAM). SAM is a degenerative arterial disease of unknown etiology that mainly develops in the bifurcations of the abdominal aorta. It requires immediate attention because the formation of aneurysms due to SAM can cause rupture and sudden intraperitoneal cavity bleeding. The prognosis for visceral artery aneurysms is poor, with a mortality rate of approximately 50% for cases involving pancreaticoduodenal artery aneurysm rupture. However, we believe that our experience is remarkable, as we saved our patient's life with conservative treatment involving coil embolization. Duodenal mucosal edema due to ischemic changes after coil embolization was observed, but this condition improved with conservative treatment.