We report a man who developed brain and bone metastases 6 years after resection of recurrent thymoma. The patient underwent surgery for B2-thymoma [World Health Organization (WHO) classification] without microscopic capsular invasion at 50-year-old. The next year, he underwent the second surgery for recurrent B2-thymoma as pleural dissemination. Seven months after the second surgery, he developed recurrence of pleural dissemination. The patient refused any further aggressive treatment, including surgery, chemotherapy, and radiotherapy. The pleural disease did not increase over 6 years, then suddenly enlarged. Thereafter, the patient developed left hemiparesis due to brain metastases, followed by bone metastases. Immunochemical studies of the metastatic tumors demonstrated that these lesions seemed to be poorly differentiated thymic carcinoma (small cell carcinoma) on WHO classification. We concluded that the thymoma transformed to thymic carcinoma with brain and bone metastases during 6 years.