An asymptomatic 47-year-old man was found to have an anterior mediastinal nodule on an annual chest X-ray. Chest CT showed a 2cm nodule in the left anterior mediastinum. Because thymoma was suspected, total thymectomy was performed. Histologically, the greater part of the nodule was composed of hyalinized fibrous tissues and there was a scattered, small aggregation of spindle to oval cells, showing positive for AE 1/3. Therefore this nodule was diagnosed as sclerosing thymoma, which is a subtype of thymoma first reported in 1994. Since few reports of this disease have been made so far, needless to say its clinical features or pathogenesis, even its very existence is not well known. Sclerosing thymoma should be taken into account, when only fibrous tissues are obtained by biopsy in patients with anterior mediastinal tumors.