An 83-year-old man presented with a mass accompanied by calcification that was found on chest computed tomography during a postoperative check. A diagnosis by a transbronchial aspiration biopsy using convex bronchoscopy was malignant fibrous histiocytoma. The histological findings demonstrated spindle-shaped tumor cells with nuclear pleomorphism that were negative for immunostain of cytokeratin, smooth muscle actin, and S-100, except KP-1. He received only radiotherapy (45 Gy/15 Fr) because of his poor performance states. However, the tumor developed mesenteric metastasis rapidly with ascites. He died on the 26th day of his second admission to our hospital, and an autopsy was carried out. The final pathological diagnosis was primary extraskeletal osteosarcoma in the middle mediastinum. For a correct diagnosis, sharing clinical information with pathologists is important.