A 72-year-old man who had undergone an operation for cholecystocholedocholithiasis 30 years ago was referred to our hospital because of abnormal findings on chest radiography. His chest computed tomography (CT) revealed an abnormal mass with a 45-mm diameter in the hepatic flexure of the colon. A foreign body was suspected and we performed tumorectomy for diagnosis. We found that the mass was present in the transverse mesocolon. Because it adhered to the transverse colon and pancreatic head, we resected the mass with the portion of the transverse colon to which it adhered. Postoperative pancreatic fistula and anastomotic leakage were observed and treated using drainage alone. Histopathologically, the mass was filled with coagula, with many fragmented surgical sutures in the fibroid wall region of the mass. Therefore, we diagnosed an iatrogenic mesenteric artery aneurysm due to the previous operation. Aneurysms of the mesenteric artery are usually diagnosed following intraabdominal hemorrhage or gastrointestinal bleeding due to aneurysmal rupture. This case is rare because it was asymptomatic and was incidentally detected via CT.