A rectal cancer was found in a 67-year-old man with a history of neurofibromatosis type 1. A low anterior resection was performed, and he received concurrent chemoradiation for 6 months. Twelve months after the surgery, a tumor was found at the anastomotic site by positron emission tomography-computed tomography and colonoscopy and was mistaken as anastomotic site recurrence. The tumor was confirmed as an inflammatory myofibroblastic tumor through transanal excision.