A 74-year-old asymptomatic female presented with an anterior mediastinal mass incidentally discovered on a routine chest X-ray. Systemic evaluation demonstrated no metastatic lesions. The patient underwent an extended thymectomy via median sternotomy on suspicion of a thymoma. The tumor had arisen from the left half of the thymus without a pedicle and had directly invaded into the left lung and pericardium. The tumor was resected with the entire thymic tissue, and the invaded lung and pericardium were resected en-bloc. The size of the tumor was 5.3x4.0 cm. A disseminated lesion on the mediastinal pleura was also resected. Histopathologically, the lesion mainly consisted of non-atypical spindle-shaped tumor cells in a so-called "patternless pattern" with various densities of collagenous background. Pleomorphism and mitoses were not significant. Immunohistochemical analysis revealed mesenchymal positive markers such as vimentin and CD34. Epithelial markers such as CAM 5.2 and AE1/AE3 were negative. S-100 protein and desmin were not stained. Solitary fibrous tumor of the thymus was diagnosed histologically. Postoperative adjuvant chemotherapy or radiotherapy was not undertaken because the benefits were uncertain. She is well without recurrence 3 months after the operation.