We report herein the case of a 28-year-old woman who presented with a mediastinal mass, subsequently confirmed to be idiopathic mediastinal fibrosis. Preoperative chest computed tomography (CT) showed a noncalcified mediastinal mass and surgery was performed to exclude malignancy. The mass was hard and dense, involved the left phrenic nerve, vagus nerve, and left upper lobe, and surrounded the subclavian artery, subclavian vein, superior vena cava, and left pulmonary artery. Pathologic examination showed the findings of mediastinal fibrosis and the mass was partially excised. Postoperative medical treatment was performed with prednisolone and tranilast, and a 3-year follow-up has not demonstrated any complications.