A 37-year-old woman was referred to our institution for further management of a mass lesion located in the thoracic cavity. The mass had grown by more than 10 cm over the course of a year and was initially considered to be a scar from previous pulmonary tuberculosis at another hospital. The patient had complained of left-sided flank pain for a year and experienced dyspnea for one month. Chest radiography and chest computed tomography revealed an irregular-shaped mass in the left mid to lower pleural cavity. The mass was widely excised through left thoracotomy. Pathologic examination of the biopsy specimen revealed a malignant spindle cell tumor, which consisted of components of osteosarcoma, pleomorphic sarcoma, and leiomyosarcoma. The patient underwent adjuvant chemotherapy and has been doing well without any evidence of recurrence for 14 months.