A 64-year-old man was found to have a 15-mm tumor in the pancreatic tail by CT angiography 1 year after resection of a left renal pelvic tumor. Clinically, the tumor was preoperatively suspected to be autoimmune pancreatitis. However, surgical resection was performed under a diagnosis of pancreatic ductal cancer, because atypical epithelial cells were detected by endoscopic ultrasonography-guided fine needle aspiration (EUS-FNA). Pathological examination of the tumor revealed a mass-forming autoimmune pancreatitis. Mass-forming autoimmune pancreatitis is often difficult to preoperatively differentiate from pancreatic carcinoma. Response to steroid treatment and the detection of extrapancreatic lesions may contribute to an accurate diagnosis, thereby avoiding unnecessary surgery.