A 25-year-old male patient of invasive thymoma was referred to our institute on March'95. He had night coughs and on the annual chest X ray exam a large tumor was found in antero-superior mediastinum. At first his tumor was too large to be resected and left brachiocephalic vein was completely occluded. By needle biopsy it was proved to be thymoma, therefore, preoperative chemotherapy was planned. The tumor responded well to the two courses of ADOC (ADM, CDDP, VCR, CAP) therapy and the tumor seemed to become resectable. In June operation was performed and the tumor was resected completely. It appears that in case of stage Ill invasive thymoma, preoperative chemotherapy is one of the choices although it seems unresectable.