A 41-year-old male visited with chief complaints of chest pain and cough. The patient was operated on under a diagnosis of invasive thymoma. The tumor was located in the left lobe of the thymus and measured 6 X 9 cm. Since the tumor invaded the left innominate vein, pericardium, and the upper lobe of the left lung, we resected it with adjacent structures. Postoperative pathological examination indicated squamous cell carcinoma of the thymus. Because the tumor metastasized also to the mediastinal lymph nodes, the patient was treated postoperatively by radiation therapy and adjuvant chemotherapy mainly with CDDP. The patient is well presently 2 years after operation without signs of recurrence.