A 66-year-old man was admitted to our hospital as a result of an abnormal shadow which was observed on his chest X-ray. A chest CT scan and chest MRI revealed an upper mediastinal tumor. The tumor was located in the retrotracheal region and resected completely following a thoracotomy. Histological examination of the tumor revealed follicular adenoma of the thyroid gland. Complete mediastinal goiter is a relatively rare lesion, with only 59 cases (including our case) in Japan prior to 1997. Only two cases were reviewed, in which the tumors were located in the retrotracheal region, out of the 59 cases in Japan.