The treatment for desmoid-type fibromatosis involves surgical resection and medication therapy, but the standard treatment has not yet been established. In the West, the usefulness of radiation therapy has been reported. We encountered a patient with desmoid-type fibromatosis in the pelvis who was treated by radiation and medication therapies and achieved a good tumor reduction effect. The patient was a 70-year-old man. He had a 6-year history of pain in the right leg and had a palpable mass on the right side of the anus; he was admitted to our department. CT showed a 12×7×12 cm mass in the pelvis, and CT-guided needle biopsy revealed a desmoid-type fibromatosis. Because tumor exclusion resulted in obstruction of the rectum, radiation therapy(60 Gy in 30 Fr)was started after performing transverse colon colostomy; simultaneous medication therapy with a COX-2 inhibitor and the anti-allergic agent tranilast was administered. Cystic degeneration was observed 5 months after the end of radiation therapy, and after 12 months, the tumor volume had halved. Around 28 months after the end of radiation therapy, medication treatment has been continued with slow contraction.