Thyroid oxyphilic cell adenoma, which is also called oxyphilic cell tumor, Hurthle cell tumor or eosinophilic cell tumor, is follicular adenoma. It is diagnosed when oxyphilic cells comprise more than 75％ of a lesion. Thyroid oxyphilic cell adenoma has been reported to comprise from 1.5％ to 10％ of all thyroid adenomas. There are varied clinical feature and malignancy, and both treatment and prognosis are unclear. We report herein on a case of oxyphilic cell adenoma which was diagnosed by chance during the examination of invasive thymoma and was resected via a thyroid lobectomy.