The patient was a 78-year-old man with a history of smoking 5 or 6 cigarettes per day for 40 years. A chest X-ray taken during a medical checkup demonstrated a mass shadow in the right lower lung field. The patient was diagnosed with squamous cell carcinoma by bronchoscopic brushing cytology. A right lower lobectomy and a lymph node dissection were performed under a diagnosis of squamous cell carcinoma of the lung (cT2N0M0, stage IB). Gross examination of the resected specimen demonstrated an ovoid, irregularly bordered, yellowish-white tumor, 8 cm in diameter, containing a necrotic cavity. Histopathological examination showed diffuse proliferation of spindle-shaped tumor cells intermingled with areas of well-differentiated squamous cell carcinoma with definite keratinization. Areas of cartilage and bone were observed in the spindle-cell components of the tumor. These findings led to a diagnosis of carcinosarcoma (pT2N0M0, stage IB). The patient developed multiple intrapulmonary metastases 5 months after surgery and died of respiratory failure 10 months later. Herein we report a rare surgical case of lung carcinosarcoma.