We report a patient with KL-6-producing invasive thymoma. A 58-year-old man was admitted complaining of dyspnea and fatigability. Computed tomography of the chest revealed interstitial pneumonia and an anterior mediastinal tumor. The tumor was surgically extirpated and diagnosed as invasive thymoma. Serum KL-6 levels later increased further and another tumor was found in the liver. That liver tumor was resected and histologically diagnosed as a metastasis of thymoma. Following resection, the serum KL-6 level decreased. Tumor cells of both primary and metastatic lesions exhibited positive reactivity to immunohistochemical staining for KL-6. A review of this case is presented.