A man in his 50s who presented an abnormal shadow on chest X-ray was diagnosed with posterior mediastinal tumor that had grown compared to the previous chest X-ray. Computed tomography showed a 5.7×3.9 cm solid mass with a smooth surface in the posterior mediastinum. A neurogenic tumor was suspected, and the mediastinal tumor was resected through thoracotomy because it was strongly adherent. The postoperative course was good, and he was discharged from the hospital on postoperative day 3. Contrary to preoperative expectations, the tumor was pathologically diagnosed as a desmoid tumor. After 6 months postoperatively without any complications, no recurrence was observed.