A 69-year old man presented with severe epigastric pain for 1 day. He had early gastric cancer at the antrum and underwent a distal subtotal gastrectomy and Billorth II gastrojejunostomy one month later without any post-operative complications. Radiologic examination revealed a large amount of retroperitoneal free air formation. Because of unremitting pain and unstable vital sign, exploratory laparotomy was followed. During the operation, a perforated duodenal diverticulum at the posterior wall of the 2nd portion of the duodenum was identified. He underwent diverticulectomy and primary closure. He was discharged on the 18th post operative day and has been followed up without any evidence of comlpication for several months.