We report herein the rare case of a 51-year-old Japanese woman who underwent thoracotomy for a mass in the mediastinum which was found to be a benign localized fibrous tumor, otherwise known as a mesothelioma. The clinical behavior of this tumor is thought to be unpredictable, and therefore we emphasize that long-term follow-up of patients with localized fibrous tumors is necessary, even if the histological features are benign.