An idiopathic mediastinal fibrosis is a rare disease with an unknown etiology. It is a benign condition in which a fibrosis of the soft tissue and chronic inflammation occurs within the mediastinum. This leads to a constriction and obliteration of the adjacent mediastinal structures, particularly the great veins. This can result in a variety of clinical conditions depending on the anatomic location of the disease. Here, we report a case of an idiopathic mediastinal fibrosis with vocal cord palsy, which was confirmed by a biopsy with a thoracotomy. Postoperative medical treatment using prednisolone and tranilast was performed.