We report a patient with a desmoid tumor arising from the mesentery and treated using high-dose tamoxifen and sulindac for about two years. A 72-year old man followed up after abdominoperineal resection for advanced rectal cancer followed by partial resection of the right lung for metachronous lung metastasis was found in CT to have an intraabdominal mass suggesting a metastatic tumor. Intraoperatively, we found that the tumor arose from the mesentery of the small intestines. Because resecting the tumor would have led to short bowel syndrome, we conducted only an excisional biopsy. Pathological diagnosis showed the tumor to be spindle-cell-like, but not metastatic carcinoma. Immunohistochemical analysis showed KIT (+), CD34 (-), and AE1/AE3 (-), leading us to start treatment for GIST using imatinib mesylate, but the tumor continued to progress. Additional immunohistochemical analysis showed β-catenin positive in the nucleus, so we have continued treatment with sulindac and high-dose tamoxifen under the diagnosis of a desmoid tumor, and the tumor has decreased significantly in size in the last two years.