We present a case of a 65-year-old woman whose thyroid cancer metastasized to the lesion of primary lung cancer. Ten years after total thyroidectomy for thyroid cancer, chest radiograph by medical check-up demonstrated three nodular lesions in the bilateral lung fields. Segmental resection of the left S6, partial resection of right S4 and left S10 were performed to remove those lesions. Histologically, small nodules in the right S4 and S10 were diagnosed as a metastatic tumor of thyroid and well differentiated adenocarcinoma, respectively. Left S6 lesion 1.5 cm in diameter was also diagnosed as well-differentiated adenocarcinoma (Noguchi type C), however, small metastatic foci of papillary adenocarcinoma was identified within the lesion which revealed to be "cancer in cancer metastasis". Metastasis of cancer to another primary cancer is a rare event. We discuss interesting phenomenon of cancer in cancer metastasis with a review of the literature.