We report a 60-year-old man, who was admitted to the hospital with complaints of cough and sputum. His chest x-ray showed an abnormal mass in the right upper lobe. After admission he noticed the painful gingival tumor. Right upper lobectomy and resection of the gingival tumor were performed. Their histological features showed that the tumor consisted of a papillary and tubular adenocarcinoma mixed with a component of spindle cells. Immunohistochemical study demonstrated a positive reaction in the epithelial component for keratin and epithelial membrane antigen, and not only these epithelial markers but also vimentin were expressed in some spindle tumor cells. Electron microscopic study confirmed the biphasic pattern, showing gland formation and undifferentiated cells. We diagnose this case as adenocarcinoma of the lung with a spindle cell component and the gingival tumor was metastatic. Autopsy showed that metastatic lesions were found in the left adrenal gland and in the left kidney. The tumor cells in the left adrenal gland were composed of spindle cells and the tumor in the left kidney showed gland formation. Immunohistochemical and electron microscopic findings of surgical and autopsy specimen suggest that this tumor is of epithelial origin, and the spindle cells are derived from immature mesenchymal cell transformation of epithelial cells.