A 51-year-old woman, who had undergone thymomectomy for asymptomatic noninvasive thymoma 12 years before, was admitted to our hospital with blepharoptosis caused by myasthenia gravis. A chest computed tomogram (CT) showed an abnormal shadow in the right lower lung field and CT-guided needle biopsy revealed findings of a thymoma. We performed extended thymectomy and partial resection of the right lung using three different approaches, via a cervical incision and bilateral video-assisted thoracoscopic surgery. The resected specimen was a lung metastasis of thymoma, and the residual mediastinal tissue showed no sign of malignancy. Because thymoma and post-thymomectomy myasthenia gravis can both recur, we recommend performing extended thymectomy or thymothymectomy, even for patients who are asymptomatic.