Carcinoma showing thymus-like elements (CASTLE) is a rare neoplasm arising from the thyroid gland. This tumor is thought to originate from ectopic thymic tissue or remnants related to thymic development in or adjacent to the thyroid gland and usually located in the lower two-thirds of the thyroid gland. The patient was a 66-year-old man admitted to our hospital with rectal carcinoma for surgery. He had undergone a right upper lobectomy of the lung for the pulmonary tuberculosis 35 years previously. He underwent a chest computed tomography (CT) pre-operatively, and an incidental nodule of the thyroid gland was detected. Based on aspiration cytology of the nodule, the lesion was suspected to be an anaplastic carcinoma. He underwent a low anterior resection and thyroid lobectomy for a double primary neoplasm. On the final pathologic examination, the thyroid lesion was shown to be CASTLE. Therefore, completion thyroidectomy with lymph node dissection of the central compartment and radiotherapy were performed, and there was no evidence of recurrence 15 months postoperatively.