A 41-year-old man had complained of the left submandibular lymphadenopathy and swelling of bilateral palpebrae. Chest CT scanning showed mediastinal lymphadenopathy and a posterior mediastinal tumor around thoracic vertebrae. Pathological findings of both percutaneous biopsy specimens of the left submandibular lymph node and the posterior mediastinal tumor revealed lymphoplasmacytic infiltration. Furthermore, many IgG4-positive plasma cells were demonstrated in the lesion of both biopsy specimens immunohistochemically. Serum IgG4 concentration of the patient was 3,660 mg/dl (reference range, <70 mg/dl). We diagnosed both the posterior mediastinal tumor and the left submandibular lymphadenopathy as a part of IgG4-related disease and treated the patient with corticosteroid, which diminished the posterior mediastinal tumor and the bilateral palpebral swelling. This is a case of biopsy-proven IgG4-related disease with a posterior mediastinal tumor.